gnuchess tries to store its gained knowledge into book.dat. Because
book.dat is not writable for common users and is located in a potential
read-only directory, gnuchess will not have a chance to evolute.

I suggest to
- move book.dat into /var/lib/games/gnuchess which is writable by group
  'games' and
- set the SGID bit on the gnuchess executable and make its group 'games'


In the current manner, the book.dat is completely senseless because
gnuchess does not use it:

--------
$ strace -eopen gnuchess
...
White (1) : go
Thinking...
open("/usr/share/gnuchess/book.dat", O_RDWR) = -1 EACCES (Permission denied)
 no book (/usr/share/gnuchess/book.dat).
...
---------

With my suggestion you'll get

---------
$ gnuchess   ## strace not applicable because it resets SGID
...
White (1) : go
Thinking...
Read opening book (/usr/share/gnuchess/book.dat)... Opening database: 5872
book positions.
In this position, 2 moves are book moves:
---------