Do you allow gaming source material -- alternative campaign setting books and the like -- into your game when only one player has access to it? For example, a player brings "Swashbuckling Adventures" to the table and asks if he can play one of the character classes or use the listed feats, but no one else in your group has had a chance to buy the book or peruse it at the store. Do you request that he share the book with others, or just play it on a case-by-case basis?