I like a lot the progress book idea.

All your experience, encounters and findings accessible from it.. it's good! Better if you could own it instead of Cain, however.
About the formulas, instead, I'd prefer they wouldn't be needed.
So no more breakpoints (+20% will add.. 20%!), no absurd exceptions (Prevent Monster Heal doesn't work on merc?), no wrong results in the Character Screen.. all more intuitive and clear! The game will teach us exactly and in simple words. We don't need a Diablo math book to play the game well -_-'
For example, about a special attribute like Deadly Strike, Cain would say "that is an incredible ability that will double the power of some of your weapon strikes". You'd see just the % on it and the long description would be inside the book. Who explains that in D2??