In some respect, one can consider a database with complete games as an opening book to about 100 or 200 (or infinite) moves.
Indeed, you can control this by changing the max-ply parameter. If you omit this parameter, every position in the pgn database is included in the resulting bin file, if you specify the parameter with a certain value n (n = 1, 2, ...), only the positions resulting from the first n halfmoves of each game are included.

Note that the resulting bin (Polyglot) file works merely as an indicator, whether a given move should be marked as a book move or not in ChessAnalyse (or other chess GUI software). This is accomplished simply by looking up the position resulting from the move in the bin file. If the position is found, the move is marked as a book move in ChessAnalyse, otherwise not.

To create opening books for ChessAnalyse it makes sense to adjust the max-ply parameter to about 40, because the resulting bin file should be kept as small as possible for performance reasons and the very most games run out of book well before move 20.

Does it work as an opening book to about 10 or 20 moves ? or more like a database with complete games?