I don't really understand what you mean by the "mental side" of the game. But for books that are a little less technique based but a little more cerebral I would check out Royler Gracie's "Submission Essentials" (that's the one with Royler and Helio Gracie in it) and while it does have a good number of techniques in it, Royler and Helio both write throughout the beginning of the book about certain qualities that you must develop for becoming a finisher and getting the tap.
However, if we're talking about pure warrior ethos, I mean really getting into the meat and potatoes of the mental side of combative sports, I would check out Musashi Miyamoto's "Book of Five Rings". It is a very skinny little book but it is just DEEP. I've been reading it since I was 15 (I'm nearly 28 now) and it is so mind blowing that you really do learn something new each time you pick it up.
Honestly the mental side of grappling to me differs very little from the mental side of boxing, wrestling, kickboxing, or any other type of combative sport.
I would suggest checking out some Japanese literature like "The Book of Five Rings"
The Book of Five Rings - Wikipedia, the free encyclopedia (if you pick it up by the translation by Steve Kaufman, I've read literally every English translation of this book and his is the best) or you could check out "Hagakure" by Yamamoto Tsunetomo (also a great book).
That's the only advice I can really give you, but if I were you I'd go with Kaufman's translation of "The Book of Five Rings". It really is an incredible book that profoundly influenced my development as a martial artist and grappler. The author, Musashi Miyamoto, lived in a time in feudal Japan where men lived or died by the sword (Musashi himself killed over 60 men in duels) and he wrote about what enabled him to win so many fights from a mental/spiritual stand point in the later years of his life. One thing he said that has always stuck with me is to, "Embody the spirit of winning by having the spirit to win".
