The preferred book format of GnuChess is now the polyglot book format 
which is supported by most FOSS chess programs and which is
documented here 

http://alpha.uhasselt.be/Research/Algebra/Toga/book_format.html

There are many free polyglot books available on the web.

This directory contains two scripts to generate polyglot books using GnuChess.


  ./make_gnuchess_book

recreates the classic GnuChess book in polyglot format. The source for
the GnuChess book is too large to put in this download but it can be
obtained here

http://ftp.gnu.org/pub/gnu/chess/

Since generating the gnuchess book takes rather a long time it 
is also available here

http://alpha.uhasselt.be/Research/Algebra/Toga/gnuchess-release/gnuchess.bin



  ./make_book <pgn-file>

generates a polyglot book from <pgn-file>. The standard player and
depth filters are not applied. There are plenty of utilities on the
web that allow one to pre-filter a pgn file.