What are circulating books?

Excuse me, but could you clarify what you mean by "circulating books"? Are you referring to physical books that are available for borrowing in libraries, or perhaps a term related to the digital realm that I'm not familiar with? If it's the former, circulating books simply mean the books in a library's collection that are currently available for patrons to check out and read. On the other hand, if this is a term unique to a specific industry or context, I'd appreciate if you could elaborate further. Thank you.