It can be hard to keep track of them (the books, not the shelves). My Aunt has a great solution.
You know those address labels you often receive as a “free gift”?
I just got a new sheet from Better Homes and Gardens to entice me to renew my subscription for the low price of way too much (Amazon regularly offers 1 year subscriptions for $5).
I got rid of the offer but kept the stickers.
Put one address sticker in each book. I usually put them on the inside cover even though some have a line to write your name in. It’s easier to find and looks neater than scribbling with a pen