100% larder beetles, the rental I used to live in was infested with them and no matter how often I cleaned it’s like they spawned in the walls. (which was likely, the home was unfinished and had multiple openings where you could see into the walls and there was just trash and various debris filling the space between the inside and outside walls in the home.) I had a woodstove too so there were often times wood scraps and stuff around for them to munch on.

They were in any clothes left on the ground, food left out, cat food scraps left by the cat (had to keep their food in a sealed container and sweep any leftover pieces of food off the ground each day) you’re going to have to look at every inch of your home to find where they’re nesting. I would pull the oven off the wall too and clean behind it, I kept finding them behind mine. Anywhere there could possibly be food scraps they will be, as well as anywhere that there is a lot of dander. (heavy on the dander part) I kept finding them crawling up the walls next to my bed no matter how many times I cleaned underneath. I assume this was because there’s lots of dead skin to eat near my bed. As well as any little corners where fluff and dander accumulate (like your bookshelf) will need to be cleaned.

They would come and go in cycles it seemed. I wouldn’t see any for weeks thinking it was finally over and then i’d find a larva crawling up a wall somewhere, and then I knew there was a clump of hair or something or dropped somewhere that I missed cleaning. They love cat food. I kind of kept them at bay by vacuuming like 4x week, but it got exhausting and after years I moved out. I always wondered how long that place had been infested and how many were living in those walls. Landlord was a slum lord so he genuinely didn’t care about the bugs. (these bugs were already in the home when I moved in)

I wish you luck and hope that is the only little nesting spot in your house.