You can read the book without having seen the show or the movie, but I think everything will make more sense if you have seen both. Also, if you're going to watch the movie, do so before reading this book because the book contains spoilers for the movie. It also contains spoilers for the TV show, so basically, watch everything VM before reading this book if you don't want to be spoiled for anything in the show or the movie.
I enjoyed this book enough that I'll probably invest in the series (however many books that may be).
It felt mostly true to the show/movie. The cameos and little inside information is fun. The mystery was solid enough. The story lagged a little for me in the middle, but picked up at the end.
There was a weird mix of informed and offensive material in the book though. Like, a pineapple pizza wasn't called Hawaiian and neither were the shirts any of the guys were wearing at the tiki party. But then basically everything else at the tiki party was cultural appropriation/offensive. There's also a white kid with dreads (unnecessary). I'm sure there are others that I'm forgetting.*
If you enjoy Veronica Mars and need a break from rewatching the series, give The Thousand Dollar Tan Line a try.
*I remembered: There's some equating gender with genitalia which is also a big No.