🚀 go-pugleaf

I can partially answer these questions.

> 1.  What is the source of the opening book in GNU chess?  
> 
The current book is based on a large collection of master games, but I don't
know details.  Perhaps Stuart Cracraft will post an answer.

> 2.  Is the format of the opening book with the current gnuchess the same as
> the format for the DOS version (version 4.0 pl 62?) ?  The problem is that
> the DOS version has a limited book.
>
The ASCII version of the book (what you get by running gunzip on
gnuchess.bk3.gz) is basically the same format, but it's possible there are
extra comments in it or other things that might confuse the book-building code
in pl62.  Hasn't anyone done a compile of the current patchlevel for DOS?

> 3.  What is the relationship between the files eco.pgn and gnuchess.bk3.gz?
> (Besides one having a lot more information and being compressed.)  Is eco.pgn
> a subset of gnuchess.bk3.gz?
> 
The files are not related except in that they both relate to the opening.
eco.pgn is not a subset of gnuchess.bk3.gz.  See next answer.

> 4.  What do the codes in the files mean?  For example, the first (non-blank)
> line of the file eco.pgn is:
> 
> [ECO    C99 Ruy Lopez: closed, Chigorin, 12...c5d4]
> 
> What does the C99 mean?  Does it refer to something from the original source
> (see question 1)?  I know what Ruy Lopez is.  

ECO stands for "Encyclopedia of Chess Openings."  This is a standard chess
reference.  It has a system of code numbers (and names) that covers all chess
openings, with more detail for the commonly-used ones.  The code number C99
means that this is the 99th opening in volume C of ECO.  (ECO is in five
volumes, designated A-E.)  eco.pgn gives the name and characteristic moves for
each code number, in support of GNU Chess's "eco" feature that identifies the
current opening.

	--Tim