This was one of the recommendations in Dani Zweig's Belated Reviews, and I remembered that I'd bought a copy in a charity shop a while ago with a load of other "Doc" Smith novels.
It's set in a universe where most of the planets and moons in the solar system are occupied by humanoid and non-humanoid aliens, but the inhabitants of Earth haven't got very far -- and when a beaten-up liner on a passenger flight to Mars is attacked by an unknown force, it's up to the crew to get the word out to Earth. There's a coherent story, but it's made up of several unconnected sections; where a modern novel would interleave the parallel stories, Smith presents his sequentially, with the occasional slightly-contrived transition between plotlines. (I'm guessing the book was originally serialised.)
The reader may like to bear in mind that this was written in 1931, and is consequentially extremely dated in many ways. The men of the IPC -- and they are almost all men, with a distinct lack of strong female characters, and a lot of old-world chivalry in play -- are all preternaturally capable, and ruthless against their enemies (which, to a close approximation, are anyone who isn't humanoid).
That's not to say that this is bad or unreadable, though. While the technology is firmly grounded in 1930s electronics ("computer" is a job title, and the protagonists spend a while building a high-powered vacuum tube for their radio transmitter), it's consistently applied, technically detailed, and believable. Smith's always been good at writing space battles, and those here are much in the vein of the Lensman series, with beams and planes of energy weapons, shields, and lots of worry over conserving power. I found the portrayal of the (friendly but utterly alien) Vorkulians especially effective -- starting with a description of their majestic capital city, and "zooming in" to two officers watching TV.
The edition I've got is the 1974 reprint, which has an impressive number of typographical and grammatical mistakes in it (in particular, lots of instances of "it's" for "its", and many misspelled technical words). It's readily available secondhand. Recommended "to nineteen decimals"; inventive fiction from the master of space opera.