It's definitely RNG, but I feel pretty confident that A1 has a higher % to hit than A2 as well as A2 having a higher % than A3.

Very similar to substats always hitting the flat stat instead of the %. lmao

Yea there is no rhyme or reason to booking, its straight-up RNG what you get.  For a champ like Skullcrusher, it can be really nice when you get his A2 booked in 1 shot, or it can be maddening when 5 books later you still haven't hit the ONE thing you want.

I feel like there is some algorithm that makes the books skill up A1 A2 and A3 fairly evenly, but that may be my own RNG