I agree that that's odd, but the career just doesn't go into that level of detail.
One suggestion I've seen online (PeteyPlaysIt's YouTube video) is to make a copy of the book, and then take the copy to the library for your other Sims to enjoy.
Many of my Sims have little artisan stores selling homemade preserves/crafts or homemade herbal remedies/jungle remedies and things like that and I thought it would be nice to add their cookbooks, travel guides, children's books too but I couldn't add any for sale. I managed to place their books in the stores for customers to read but it's not quite the same. I couldn't set them for sale.
In previous versions of the Sims I'm sure you sometimes saw your Sim's book in a library.
