This is in answer to Pawel Koziol's e-mail on February 2014.
I have checked that GNU Chess 6.1.1 will not always reply O-O in the following position, with the engine book enabled:
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 3.Ba4 Nf6
It will, however, with the provided patch. The patch will be included in versions higher than 6.1.1.
Thanks again to Pawel Koziol for his kind contribution.
--Antonio Ceballos