BGG has had a fluctuating policy on books over time. The current policy is that books of rules for games should not be listed, but the individual games should be listed. For example, Scarne's Encyclopedia of Card games should not be listed, but the individual games whose rules are contained therein should be (and most or all are). Books about game strategy, or catalogs and pictorial collections of historic games are not content for the database.
This entry is to make listing, selling and trading of those other books possible at BGG. Books about one or multiple games or about more general topics such as game designing or the history of games belong here.
Note that boardgame magazines go under Miscellaneous Game Magazine.
Game catalogs, brochures, etc. go under Miscellaneous Game Merchandise.