Edmond Hamilton's "Outside the Universe" (1929)

The late 1920s saw a period in which "space opera" really established itself as a genre. E.E. "Doc" Smith's multi-volume series (Lensmen, Skylark, etc) is probably the most famous writer of early space opera, but Edmond Hamilton also created a shorter series involving the Interstellar Patrol, an alliance of different alien races from the Milky Way galaxy (the most famous modern version of this idea is Star Trek's "Federation of United Planets"). 

Hamilton's novel Outside the Universe (the last of the Interstellar Patrol stories) was serialized in Weird Tales from July to October 1929, featuring title illustrations by C.C. Senf. In 1964, the serial was collected and published as a single novel by Ace.

In this story, the Interstellar Patrol encounter an invasion force from another galaxy ("another universe"). After the vanguard force of these "serpent aliens" defeat the Patrol, the narrator's single surviving ship is tasked with seeking out help from the serpent people's old enemy from the Andromeda galaxy. However, on the way across intergalactic space the IP ship is captured by the serpent people and taken back to their dying galaxy. After a daring escape, they eventually make it to Andromeda and obtain the help they need to defend the Milky Way from these invaders. 

The book is brimming with large scale space battles and fleet maneuvering. The science is obviously dated and completely at odds with modern science, but one can enjoy it just as an adventure. It's very much as if Hamilton described a naval war between three continents, but moved it out into inter-galactic space and filled it with bizarre aliens and super-weapons. For example, although the main character is a human (the only one featured in the entire book), his most trusted ship mates are a crablike alien and a robot-like alien. In space, the races use different kinds of technology to manipulate space "ether" in order to attain faster than light speeds. Various super-weapons include "attractor (tractor beam) ships", "sun-swinging ships" (antigravity) and high radiation death beams. The Andromedans are armed with some kind of "implosion" beam. 

In any case, although the prose eventually gets a bit repetitive (these were meant to be read over several sittings spread a month apart each, after all), there is a real "kitchen sink" aspect to it, and the tactical elements of the space battles hold up nicely. 

There's an excellent review of the book here as well.

Detailed Synopsis

1. The Swarm From Space: While patrolling the perimeter of the galaxy, Dur Nal’s Interstellar Patrol ship spots a swarm of objects approaching from outer (inter-galactic) space. The swarms soon turn out to be hostile spaceships.
2. Chased Through the Void: When all other ships of Dur Nal’s patrol are destroyed by the aliens’ death beams (some kind of radiation which doesn't harm the ships themselves), he orders a retreat and radios the Federated Suns so that they can prepare a trap within the galaxy itself. He then races towards the interior. Although the alien ships are faster than his own, he briefly tricks them by leading them into an “ether maelstrom”. As the chase continues, Dur Nal navigates through a double star, after which a hundred of the pursuing alien ships are ambushed by ships of the Interstellar Patrol.
3. Death-Beam and Crimson Ray: A massive fleet battle ensues. Unfortunately, the alien ships use “attractor beams” to immobilize and destroy the ships of the Interstellar Patrol. The only ship to evade the slaughter is Dur Nal’s ship, which breaks through the enemy line in a ramming maneuver. Unfortunately, this maneuver compromises his ship’s hull. With only minutes to spare, he orders his crew to attempt to board a lone enemy vessel.
4. A Struggle Between the Stars: Dur Nal’s crew leap onto the enemy ship and once inside, destroy the serpent aliens within. They pilot the ship towards Canopus and the Council of Suns. However, when they arrive, IP ships rise up, taking them for an enemy force.
5. For the Federated Suns!: After using signal lights to alert the IP ships, Dur Nal passes out. He later awakes and learns that the Council of Suns has studied the ship’s records and learned that the serpent people come from a galaxy whose suns are all dying out. After an initial failed attempt to conquer the Andromeda galaxy, they had then decided to conquer the Milky Way. Dur Nal is given a new mission: use the captured serpent ship to reach Andromeda and ask for their help. Dur Nal and his crew head out into intergalactic space.
   
6. Into the Infinite: During a journey of several weeks, the ship contends with heat and radiation whirlpools. Eventually nearing the Andromeda galaxy, a few dozen ships approach them. Unfortunately, they are serpent ships and one of them rams Dur Nal’s ship.
7. The Gates Of A Universe: Dur Nal’s ship is captured. He and his shipmates are then taken aboard an enemy ship prisoner, as the entire force heads back to the serpent peoples’ galaxy, which lies closer to the Andromeda galaxy than the Milky Way. As they approach the serpent galaxy, they see that the entire galaxy is protected by a force shield. After passing through a gate guarded by two large fortresses, the serpent men and their prisoners debark on a planet. There they see a giant cone-shaped death beam weapon under development, which, when completed, will be able to end life on entire planets with just one shot. During a prisoner struggle, first mate Jhul Din and a few of Dur Nal’s other compatriots escape in one of the ships.
8. The Hall of the Living Dead: Those of Dur Nal’s men who fought a delaying action to permit Jhul Din’s escape are brought to a museum where they are put into a conscious suspended animation. Fortunately, Jhul Din eventually returns and breaks them out.
9. A Dash For Freedom: Dur Nal quickly frees the other (older) prisoners of the museum and their attack against their long-time captors provides Dur Nal and his allies time to make it to their stolen ship and escape. When they near the serpent galaxy's force shield gate, they quickly dodge downwards, leading the serpent people’s pursuing ships directly into the line of fire of the gate fortresses. In the confusion, Dur Nal’s ship eventually pushes past the gate portal and heads towards Andromeda.
10. Flight and Pursuit: Dur Nal’s ship is pursued as it races through space, with the faster serpent ships slowly catching up to them. They buy some time by navigating through a radiation maelstrom but ultimately are forced to make a last stand outside the Andromeda galaxy. Fortunately, unknown ships swoop in and attack the serpent people’s ships.
11. Into the Andromeda Universe: The green-gas-based Andromedans drive off the serpent ships, and make contact with Dur Nal and his crew through a thought-vision device. Dur Nal has his ship follow the Andromedans to their galaxy where they see that the Andromedans have situated their stars so that they form circles around collected planets. Dur Nal heads towards the central planet, but his ship begins to break up from the strain.
12. The Council Decides: Dur Nal’s ship is “caught” by the Andromedans and they land safely. He sees that the Andromedans’ central planet is mostly without a cityscape, since they build structures heading underground. They also have “sun swinging” ships with which they use to arrange the positions of their suns. The Andromedans agree to stage an attack on the serpent galaxy, but the serpent people make a surprise attack on the Andromedan central planet and capture their sun-swinging ships, their main advantage. Dur Nal's friend metallic friend Korus Kan is also captured while insde one of the sun swinging ships.
13. The Sailing of the Fleet: Despite the loss of their most powerful weapon, Dur Nal leads the Andromedan fleet to the serpent galaxy where they are met by the enemy fleet at their galactic gate. A massive battle takes place, until the Andromedans stage suicide attacks on the two guard fortresses. The Andromedan fleet lands on the serpent peoples’ central planet, but Dur Nal is horrified to learn that the serpent people have already left for the Milky Way, and that the defense of their gate was only a delaying action.
14. Back To the Galaxy: Dur Nal leads the Andromedan fleet back towards the Milky Way. On the way they are suddenly pulled into a heat maelstrom by the waiting attractor-ships of the serpent people. Some of the rearguard Andromedan ships escape the attractor beams and circle around to destroy the serpent ships. As the surviving fleet approaches the Cancer cluster (the serpent peoples’ foothold in the Milky Way), the enemy fleet rises out of the system to meet them.
15. An Armageddon of Universes: The two fleets engage in a massive battle. Andromedan suicide bombers gain a momentary advantage on the serpent ships. A wandering “dark star” (neutron star) passes through the battlefield, also throwing the serpent ships into confusion. However, a new squadron of serpent ships then emit “ether currents” behind them, which scatters the Andromedan fleet. On the brink of disaster, Dur Nal’s fleet is saved by the arrival of ships from the Federated Suns, newly-built since Dur Nal had begunhis trip to Andromeda.
16. From Outside the Universe!: A massive three-fleet battle erupts, with each fleet forming various strategic positions. In one phase the serpent ships begin making suicide attacks of their own, throwing all three fleets into disarray. The battlefield drifts through a double-star, causing more havoc. Eventually the serpent ships flee back to the Cancer cluster and make a last stand. This gives the serpent people in the cluster time to complete their giant cone-shaped planet-destroyer and launch it towards the Andromedan and Federated Suns fleets. Just before the cone destroys the fleet, the lost Andromedan sun-swinging ships (led by Korus Kan) swoop in and cause the entire Cancer cluster to implode on itself, destroying the cone planet destroyer in the process.
17. Outward Once Again: It is revealed that Korus Kan had used the sun-swinging ships to destroy their captors, but by the time they returned to the Andromedan central planet the fleet had already departed. The Andromedans depart back to their own galaxy and Dur Nal escorts them to the edge of his own galaxy.