The DC library has a large selection; sometimes the actual titles they choose seem strange to me, but whatever. There is usually a wait for the more recent and popular books. The wait isn't normally too long. If you want to read library ebooks, you have to be willing to wait. Often you can get a popular book quicker by walking into a library and checking it out--BUT--and I just say this as an example of course, someone at DC overdrive library bought like a zillion Harlequin romances in PDF, and there are none at all of these at my IRL library branch, so I've been working my way through this backlist
I think more recently epub titles are the favored format showing up but most of their older titles are in PDF and mobi.