I've never found too many of the d20/DnD books to be great reads, with the exception of the Iron Kingdoms books (especially the Character and World guides). I dug the Dragonstar stuff as well, though, and back in the 2nd edition days I was completely in love with Planescape.
Other rpg books I enjoyed reading are a bit far between. It's been quite some time now, but way back when Rifts first came out I picked up a copy and got wrapped up in the awesomeness of it all (well, I was about 13 or 14 at the time, so...).
In my mind, Unknown Armies 2E is quite simply the most readable rpg book ever. When I got my copy I almost went cover to cover in one sitting, something I haven't done in years. I wouldn't hesitate to suggest it to anybody, even those with no intention of actually playing the game.
I'd also give HOL: Human Occupied Landfill and its only supplement, Buttery Wholesomeness a nod. Though reading them can be challenging at times (both books are completely handwritten), they're absolutely hilarious. I don't remember reading a book that made me laugh so much. They're definitely not for those role-players that take themselves a bit too seriously, however.
