How about having some books adding synergistically or additively to certain skills?
I’m thinking situations where even a lvl 10 survivor would gain something from being able to refer to a high level book. (noone can keep everything in his mind). The bonus could be in failure% and crafting speed.
Also same for low lvl skills… E.g. Having a lvl 3-5 book on tailoring and an own skill lvl of 3 will add a small bonus when crafting/repairing clothes…
Candidate skills could be:
Tailoring, cooking, computers, mechanics, construction, first aid, electronics and trapping…