After some research, I am all but certain that this is the correct forum and subforum for this kind of thing. In fact, I am all but certain that I have seen quite a bit of discussion in this subforum about it. It won't fall under the heading of "deals," but surely it does under the heading of "resources."
"It" refers to the topic of lending libraries. Most of the discussion that I remember seeing in this subforum about it relates to brick-and-mortar libraries, what it takes to get ebook borrowing privileges from them, etc. But, I don't remember the following lending library ever being mentioned (if so, I think that it must have been quite a while back).
I don't remember anyone bringing up the Open Library's lending library, before.
I didn't see a figure for the total number of books in the library; however, it must be huge. I searched for the word "Bible, for example, and got nearly 5,300 hits (keep in mind that individual books may bear quite a few "tags," and, therefore, show up in searches for quite a few different terms; still, I think that this can give you some idea of how many total books that we're talking about). Borrowing appears to be completely _free_.
Open Library calls these ebooks "modern ebooks." Now, I don't know about you, but I don't consider books published in the late 1800's and early 1900's to be "modern books." However, I saw a few of those, especially ones from the early 1900's. By far, though, the great majority of the books do seem to, indeed, be modern--probably on the order of what a typical well-funded brick-and-mortar public library would have in its stacks.
www.openlibrary.org/borrow.