Hey First of all thank you so much for playing my game. I'll try to add full playthrough on my youtube channel and link it for you.
There are 2 ways you can reach to the book. One via golden key. One more via mining red crystals using picaxe and offering it on one of the stone near golden door.
To be honest I only completed my game on hard difficulty. I'll make some tweaks for insane difficulty
And once again i was not expecting anyone to play my game and i was laughing for a min when i saw your comment.