Using Books

A subscriber in their profile states they use books. Is this legal? Permitted?

Yes, you can refer to books and databases of games.

Rule 3. b. While a game is in progress you may not refer to chess engines, chess computers or be assisted by a third party. Endgame tablebases may not be consulted during play but you may reference books, databases consisting of previously played games between human players, and other pre-existing research materials.