The books add details that add complications and for me, unless the GM can memorize it all - which is usually me - really slows down the game.  It ruins the flow.  I run a really loose game to keep the story flowing as smooth and fast as possible, so I don't want numerous volumes of details.  I want the details to be color in everyone's heads, in the settings.  And I got a lot of compliments on the games that I ran.