Tonight, I installed the gnuchess 5.04 program into the internal memory of my Zaurus SL-C760. The gnuchess program apparently looks in the current directory for the book.dat file. So, if you go into the terminal program and type the link command that I mentioned above when the book.dat file is on a compact flash card, gnuchess will use the opening book. Alternatively, if the book is on the SD card, you can use the command:
ln -s /mnt/card/book.dat book.dat
to create a symbolic link in the current directory to the book.dat file on the SD card. I just played a game against gnuchess with the default settings and qboard 1.1 as the graphical user interface. I won, but after I checkmated, I could not save the game. Qboard informed me that gnuchess was no longer running. I then could not play another game until I rebooted. Perhaps there is a memory leak somewhere.
You can test to see if the opening book is being used in qboard by switching sides in the Options menu to play as black, then play as white, then play as black, etc. The opening moves should be recognizable.