[eudamoniac]

Well first of all if thirty books disappear in one day, that's probably an inorganic usage. If none of them ever reappear, that's another indicator. And then if the person you see taking thirty books is dressed in rags with a shopping cart, you can be pretty confident.

Or maybe OP just means that none of them ever return; it's supposed to be a LF Library after all, not a LF bookshelf.

[SamBam]

Huh, I've literally never heard of someone before thinking it's supposed to be a library that you return books to. Must be different attitudes in different places. I've always seen people treat it as a swap-shop. Take some books you want, and some other day drop off some books you want to give away.

[wccrawford]

Both are okay. Returning or just keeping, as long as you aren't excessive. You aren't supposed to just take all the "good" ones, no matter if you're keeping or selling.

And there's an unspoken rule that you should probably give back, too. It's a community good, not a charity.