This book (https://shop.gogameguru.com/relentless/) is amazing in every department: Size, content and last but not least the license (http://creativecommons.org/licenses/by-nc-sa/4.0/)
Though I started studying this game seriously only 6 weeks ago (call me AlphaGo boy) I know enough about chess, backgammon and poker literature to recognize: Relentless will be a milestone.
Ciao
acepoint