...have a big influence on an engine's tournament performance. The problem is, I never made an optimized Aristarch-book because I am mainly interested in developing a chess engine, not a chess database (if anyone has experience in opening books and likes to design an Aristarch book, he is welcome).
When comparing with other engines, I want to exclude any influence of opening books, therefore I use predefined starting positions as described in the section Testing Aristarch. Hence, Aristarch's books are probably not very good and are delivered only to make Aristarch work with free interfaces like WinBoard. With commercial interfaces you can use external books, but I never tested if the Aristarch books perform better or worse than these books.
The Aristarch books white.boo and black.boo are generated automatically by Aristarch itself. They consist of a large collection of lines where Aristarch won games. This learning can be turned on via the corresponding UCI option or the command line option "-g".
In every case the file aristarch.boo should be present, as it contains detailed opening knowledge which is used whenever the lines in white.boo and black.boo end. The large book aristarch.boo is not in the main archive, but can be downloaded in the Downloads-section.