So I know that every skill book has the same chance of success, and no feeding method, or arcane rituals, or prayers to the great Usopp, etc, is going to change that.
Regardless, as I've been farming Peronas, I've been feeding books to Dog Penguin, Kumacy, Yosaku, G2 Luffy, and Helmeppo. With Dog Penguin, I give him books/copies of himself whenever I have 3 or more. Yosaku, Helmeppo, and Luffy get theirs one at a time. Kumacy tends to drop in batches, so he's getting similar treatment to Dog Penguin. My results so far:
Dog Penguin: 1 skill level from max (so like 8-9 skill ups so far)
Kumacy: at skill level 11 (10 skill ups so far)
Helmeppo and Yosaku: 1 skill up so far (which maxes Helmeppo, had him leveled from earlier fortnights)
G2 Luffy: Nada
Obviously, Dog Penguin and Kumacy are dropping more, so they've probably had double or even triple the books as the other guys. But even with that, these are weird results. Yosaku especially, I've gotten enough books to get him to level 40, but he's only gotten one skill up in that time.
Again, I wouldn't read anything from this
but, if you want to be paranoid, feeding books in sets of 3-5 might be something to try.