If there is any one book to have, it is this one. It is the only Pathfinder book you will ever need to start playing, but if you're like me you'll eventually want more.
When I first learned of tabletop RPGs my attention turned to D&D even though I knew next to nothing about it, nor did I know of anyone who played it. I only knew it because it was the name everyone knew when someone said "tabletop RPG" and the answer wasn't "what is that?" I wanted to play it but I didn't know where to start. I was lost, forlorn, and alone.
Then, one fateful day, I met someone online who told me about Pathfinder. I took one look at the Core Rulebook and I never looked back, and to this day I don't regret the decision one bit. The Core Rulebook is a solid start to any aspiring tabletop gamer's adventure and is a must own not only for the abundance of useful information it provides but also for the clean presentation and the magnificent art provided by Wayne Reynolds.
In short, if you want to play Pathfinder and haven't already, pick this book up immediately. It is well worth it.