Oyster, a startup that offers offers unlimited access to its collection of e-books for $9.99 per month, has crossed a nice milestone — it says its library now includes 500,000 books, compared to 100,000 when it launched in September of last year.

The company also said it recently expanded its partnership with HarperCollins, bringing 10,000 more of the publisher’s titles (including Beautiful Ruins by Jess Walter, The Happiness Project by Gretchen Rubin, and American Gods by Neil Gaiman) into Oyster. And it has signed new deals with publishers including McSweeney’s, Chronicle Books, Grove Atlantic, and Wiley.