@RaichiSims I can understand your frustration, and I am happy to help in any way I can. I'm sorry it's frustrating. If you don't come up with a better solution, and your sim is short on money, you could always go into a household of an in-game (not your) sim and play that sim - take them to the library, and have them buy/stock the books you want. Hey, what else do they have to do? 😉
This will sound really weird, but if you make the library residential, move a sim in, the bookshelves should have books. If they don't, buy new bookshelves - some of the nice ones with more books, and there should be books in them. Some of the skills books may be in the more expensive shelves, I don't remember. Then if you drag all of the books to the floor and have the sim leave the lot (before anyone puts the books away), then change the lot to a library again, those books will be picked up and put away by the community sims and then they will be part of the library. You would then just need to have a sim add the skills books or any that didn't come with the bookshelves. That would cost less than providing all of the books. And, as I said, you can play a rich sim and have them go in and purchase and lay out the skills books for everyone to put away. It can be their benevolent philanthropic contribution. 😉
Having said all of that, I think you make a great point... if a bookshelf is placed on a public lot (library, etc.) it should contain the books it would normally. It would also be good if there was a way to purchase books for it during build to be used in the library when placed. Perhaps it was not an aspect of people being able to build/add libraries that the developers realized was going on? If you would like to I expect you can post this issue on a suggestions forum. @PugLove888 do you know of any for this?