I have played 7 Days to Die for a total of 70 hours (on PC), yet I've only completed a total of 6 sets of perk books across two solo worlds. It's either my bad luck, or a slight flaw in the game. A solution to this problem would be to reduce the probability of finding the same volume over and over again. For example if you were to read volume 1 a second time (or second, third, forth, etc.) it would reduce the probability to find it again in loot as opposed to other volumes of the series. I don't know how the looting system works on a technical level, but it'd be really cool not crying myself to sleep every time I see volume 5 of Tech Junkie in particular for no particular reason. Also it would be nice if traders sold you a different volume than the ones you've already read as well, though I'm unsure how that'd work in a multiplayer world.
A bit of incentive.