Right-click the book window and select Properties.
Np
The number of different positions contained in the book. White and Black are not differentiated, nor are positions mirrored on the middle axis. Try entering the moves 1. e3 e5 2. e4 Nf6 3. Nc3 Bb4 and how this becomes a reverse Ruy Lopez.
Unplayed transpositions
Shows moves that have never been played in the current position but which lead to another position known to the tree.
Retro-moves
Displays all moves that lead from positions in the tree to the current board position.
Statistics
Displays bar graphs of the move statistics, and also gives the exact number of Elo rated games.