From the title and cover I actually thought this was some sort of YA-ish romance book (oops, probably getting confused with "The Space Between Us") but it's actually a decent soft sci-fi novel.
Basically they've invented traveling between parallel worlds, but the catch is you can only travel there if your counterpart in the other world is dead. Since rich people tend to be alive in most of their worlds, this means the job of traveling falls on the poor underclass.
Our main character, Cara is one such traveler. She travels for quite mundane reasons - to gather data on parallel worlds for statistical analysis and to try and make money off of it. The book isn't too heavy on sci-fi, as it more ends up being about the people that Cara meets between the different worlds, and all the ways they are different (or the same). So don't expect the author to really explain the travelling thing.
The first quarter felt really slow as the author established the character's backstory and all the supporting characters and I was half-considering giving up on it. It did pick up the pace midway for an relatively satisfying conclusion. The villain and the way the ending played out definitely had room for improvement, but just don't read too closely into it lol.