you can build one with your software...
Free online CTG Opening Book

you could update it yourself, using games downloaded from TWIC - http://theweekinchess.com/twic
How about a Free HIARCS Opening Book Download?
Click on this link: https://www.hiarcs.com/chess-opening-book-free.htm
and look for this text:
HIARCS 12 Book in CTG format

Are there any free CTG openings books out there for download? Links?
- Maybe a book based on Elo 2500+ games?
- Maybe a book based on Elo 2700+ games?
- Maybe one that gets updated with new GM games regularly?
I don't use database of 2500-2700 people because there are database of 3500+ rated machine available now. Actually these books are built with very deep analysis, the rating and accuracy is probably 3700- 3800+. The good point is they are free and currently updating.

What does CTG mean?
A kind of book format mainly used by chessbase ( German software Giant company).
There are a lot of book formats. Another popular one is polyglot.
The main theme is GUI ( your software) must support your book format.

What does CTG mean?
A kind of book format mainly used by chessbase ( German software Giant company).
There are a lot of book formats. Another popular one is polyglot.
The main theme is GUI ( your software) must support your book format.
Does fritz 17 support opening books?
Are there any free CTG openings books out there for download? Links?