Most of our baby/young toddler books are board books and have held up well, but there are some -- the ones with pop-ups, moving parts, or flaps to lift -- where DD1 basically destroyed the functionality over time. Moving tails missing, flaps torn off, etc.
Do you replace those kind of books for later kid(s) so they get a new version, too? Leave them in circulation as is? Toss 'em? Get different books?
So far I've tossed a few, and only replaced the ones I really liked and thought were neat -- which unfortunately also seem to be the most vulnerable ones. The Matthew Van Fleet books in particular (Tails, Dog, etc.) are great, but man, they don't stand up well to curious babies/toddlers.

Re: Moms of two+: do you replace books?
Almost all of DS books are available for him to play with. There are a few that I don't allow him to have all the time for that very reason. I don't want them completely unreadable or unusable, so we have them for special reading time together.
I tend to go to the library and buy books for 10 cents, 25 cents, etc - that way I don't feel like I am throwing away 5+ dollars if a book is ruined.
So far I haven't needed to replace any. We have Dog too, and I know the wet dog's head was torn off not too long after we got it for DD1. We have a couple other books with pull tabs or pop-ups that have been destroyed, but those aren't worth replacing to me. Those books and any other books I don't mind getting destroyed are in our playroom, while our better story books are in the girls' bedrooms so they aren't handled as often.
If any favorites -- Goodnight Moon, Time for Bed, Knuffle Bunny, etc. -- were destroyed, I'd probably replace them.