For me, the best RPG books to read make me want to run or play a game (even if it would never work for me or my group).
Back in the day, Shadowrun books were great fun to read. I bought way more SR books than I used, mostly because they were so entertaining.
I also generally like Exalted books, too.
Ken Hite's (Cabal, Weird War) & David Pulver's (Technomancer, Transhuman Space, Reign of Steel) GURPS books are always entertaining.
Aaron Allston's Champions/Hero supplements -- Strike Force, the Blood & Dr. McQuark, the original Ninja Hero.
Various Mutants & Masterminds books (the core book, Freedom City) qualify, too.
In regular d20 land, Book of the Righteous, the various (Complete) Books of Eldritch Might, Midnight, and the Draconomicon.