The title might suggest the book tells the events during a journey to Andromeda. It's more about the aftermath of an unintentional trip to Andromeda. "Unintended??? trip to Andromeda?" you ask with a puzzled look. The premise is a new kind of spacecraft drive intended for interplanetary travel which gets out of control on its first runs. It not only over-performs its creator's expectations, but supposedly proves Einstein partly wrong.
The story is more about the return of the spaceship. The ship's captain has come back planning to rebuild the devastated Andromedan race on Earth. (Their galaxy, coincidentally, was doomed to be destroyed shortly after the journey back to Earth.) However, there are issues with Earth authorities, and between different Earth authorities. When the spaceship left, it disappeared as far as Earth could tell. When it reappeared, only a month has passed on Earth - while the captain claims to have been gone for decades. Some in the military view the returned ship as a threat and plan to strike against it. Some authorities don't believe the story about going to Andromeda. Others simply question the captain's plans for the Andromedan race.
There are aspects of the book that suggest pulp. It's more a story of intrigue and B movie action. I don't have high expectations for a book's writing technique, but this was not as good as I'd prefer. Probably better technique than E. E. Smith, but below average.
There may be points where the book's "science" doesn't sound stupid, but there are plenty of examples where it does. (For instance, we're told the Andromedans are similar enough to humans for the two races to interbreed. On the other hand, we're told Andromedans are so different from humans that an Andromedan crumbles into dust within a few hours of dying.)