Stockfish Polyglot is the Stockfish chess engine with the ability to read Polyglot files, also known as bin books, and it can read up to four of them. This chess engine automatically detects your maximum number of threads and set it as default value in the UCI settings.

STOCKFISH POLYGLOT CANNOT DIRECTLY READ CTG BOOKS, YOU WILL NEED CHESSBASE SOFTWARE OR AQUARIUM. Note that you don't need to fulfill all the four book slot and that you can use random slots (example 1 and 3) paying attention that the earlier gets the priority on the move choice; examples at the bottom of the page.

By enabling BestBookMove you are just allowing Stockfish Polyglot to play the move with the highest percentage for any position that is included in the Polyglot book, nothing more, nothing less; it won't guarantee a better performance. Usually I enable it when I play with different opponent on Playchess and I disable it when I make book tournaments to avoid repetitions. One more thing: with BestBookMove enabled, if you have a Polyglot book with moves having the same percentage for a said position, Stockfish Polyglot will always choose the first one.

Example: starting position with 1.e4 33%(first), 1.d4 33%(second), 1.c4 33%(third)-> with BestBookMove enabled Stockfish Polyglot will always move 1.e4.