They do have a library of stuff they own or license, too, though I personally am not interested in much of it. It's worth mentioning that some of it involves reasonable investment with celebrity readers or more expensive production. (I can't stand any of that. A second reader for different chapters is tolerable; more is not.)

It's 36 for 3 credits after that, with occasional sales of two specific titles for one credit or discounts on cash price. I'm not sure how they structure their actual deals with publishers, but I am reasonably sure that they're leveraging their market position hard to sell some of those books at those prices, because they're way less than anywhere else including other formats.

15 hours makes the whole "we include audiobooks in your subscription" to be a pretty token service, though. That's not that much time.

There is at least one actual subscription audiobook service that is close to unlimited* and has a decent library (though it's older and less known content, and discoverability it pretty bad). I've found several series I've read 10-20 books in a row of a month through scribd. (I can provide a referral for a free trial on request. Not trying to advertise though). I'm guessing Spotify is going for high profile stuff, though, and that costs more.

*How it works is that certain publisher deals will only let you listen to a certain number from an author or in a series in a month, then you have to wait until the next month for the rest. But you can still access the rest of their library.