ShinHakkaider
Adventurer
Heh, I *wish* my players would bring their books! So often I've seen a player with an apparently wrong interpretation of a power, but no book in which to check it!
As DM I always bring a PHB these days, but even with limited sources I can't fit them all into my RPG bag. I can see a case for bringing netbook + DDI subscription instead since the pubs I play in have wifi and the online Rules Compendium is very good these days, but I'm worried about breaking it while in transit through the London Underground.
When I got my iPad it's been a lot easier to cart around a whole mess of books for reference, but still when we do allow anything out of core in the game the player is responsible for making sure that they have the information handy for reference during the game. Usually I just point them toward the PFSRD and to copy stuff to a word document and print it out for easy reference.
It's understood that if they dont have it handy for quick and easy reference then it's not getting used in play. That way if takes the burden off the DM and places more responsibility on the player for their character, where it should be. It's their character they should know how their own fiddly bits work...