I actually found all the books a decent read (core 3, MP,
FR Players guide, AV). But I enjoy reading rules, it sparks my imagination. Of all the stuff in the books, the hardest thing for me to enjoy reading were the 2 page area descriptions in the FRPG.
Part of that is that Hero is my primary love in RPGs... I read the Hero 5th ed rulebook (a completely setting free book of rules) and enjoyed it thoroughly.
The second is that I get inspired differently than others (from what I see). When I read rulebooks, like the PH, Adventurer's Vault or Martial Power, I'll see a new power, a new feat, a new piece of equipment and my mind will almost instantly see a character/event/plot with that - with background and story potential showing fully formed in my head. Rules inspire me, so I find them really fun to read.
Fluff on the other hand usually leaves me cold, I read it and it is just like listening to someone drone on about their character, their world ect.... boring.
So I will be in the minority who say I really loved reading the rulebooks, and have read the ones I have cover to cover like a novel.