"Nebula Alert" is a 1967 science-fiction novel by A. Bertram Chandler, published as one-half of an Ace Double, with the other side being The Rival Rigelians, by Mack Reynolds. "Nebula Alert" takes place in Chandler's "rim world" shared universe, more or less.
Irene is an ex-galactic empress and space freighter owner who is married to the captain, Trafford, which seems like a good backstory, this book being the final book of a trilogy. All of this backstory is hardly mentioned once we get underway. The ship has been hired to transport a race of aliens who are being targeted by slavers for their special abilities. This pursuit drives them close to the Horsehead Nebula, which interferes with their warp drive (which isn't called a warp drive, but it is a weird thing that allows Faster than Light travel). After drifting through the Horsehead Nebula, they find themselves in an alternative universe, much like theirs, but with an altered history. There, they meet John Grimes, the main hero of Chandler's work, who was the protagonist of such works as The Road to the Rim, who lets them know that they have entered a parallel universe. They figure out a way to send the ship back through to their own universe, and evade the pursuing slavers. They then take a trip back through the nebula, where the book reaches its climax, in a battle that I didn't quite understand.
Incidentally, this book has the tagline "The impossible route to a rebel universe"...which, in true Ace fashion, doesn't really describe the plot. The cover shows a domed city with tailfin rocketships, none of which relates to the story. It is basically a generic science-fiction cover.
Which brings me to my main point of this book: it has some interesting concepts, such as the alien race that is programmed to be immune to slavery because they will die if they leave their planet for too long, and the idea of nebulas as being outflows of particles from other universes...but in many ways, this book is very standard space empire, rocketship science-fiction. Even though it throws in a lot of concepts, it is basically solid space opera that the reader will probably be familiar with.