If you don't own a book, ask the player who wants to play a non-core class to let you borrow the book for a week, along with a copy of their character sheet, to familiarize yourself with the class concept and how its abilities work. Make a few notes for the more complicated concepts. For small things like feats or individual spells, it's usually not too hard to review those things piecemeal. Also check and see how much of the info might be available online ... sometimes the stuff players want to use will be available as a preview or web enhancement straight off the WotC web site. For example, every single ToB maneuver and stance is available via the maneuver cards ToB web enhancement.