sims put books into their inventory

I have the same issue. When it's one of the sims you're playing, you can just put the books back; annoying, but easily fixed.

However, I've had a sim (npc) stay the night and he woke up in the middle of the night and started browsing books. So he grabbed a book, read a little, sat down and put the book in his inventory (basically stealing the book). And this happened over and over again (he is a bookworm apparently)...

(I don't know if its relevant, but in this case the 'bookshelf' was the item "Vintage books", the row of books, placed on a desk)

Interesting... I actually liked when my sims held onto books when they were skill books I was trying to get them to read frequently... but some of them definitely do this and I can see the issue if an NPC does. I also had one of my toddlers randomly pick up a book one time (on rocket science, I think- it wasn’t a kid book), which was very weird and made me think I’d lost the book for a few sim days until I noticed it while checking inventories for laundry and dirty plates.

More annoying to me though, is when they just put a book (or their homework) down on the floor, regardless of where they are. So my sim may have stashed a skill book I bought them in their inventory, but then when I take the whole family to the snow festival or something I have to keep a close eye on them because they may autonomously pull it out to read then leave it on the ground when they finish. Or they may even technically still be “home” but have wandered far from the house while I was distracted and dropped it in a nearby park.

I suppose that wouldn’t be an issue if they just always returned books to the same bookshelf they got them from.

I realized at one point that my Sims had completely looted the bookshelf of their favorite vacation rental; it contained a single book! I’d wondered where all the duplicates in their inventory came from. I ended up buying the vacation rental more books. 

 I’ve noticed before that non-kleptomaniac Sims will read books or play with a slablet and pocket them.  I once rushed off to play Cassandra Goth so I could make her return my Sim’s slablet!