Heather Morrison has published "Dramatic Growth of Open Access March 31, 2016 " in The Imaginary Journal of Poetic Economics.
Here's an excerpt:
There are now 150 publishers of peer-reviewed open access books listed in the Directory of Open Access Books, publishing more than 4,400 open access books. 620 books were published in this quarter alone, a 16% increase in just this quarter. The Directory of Open Access Journals has been adding titles at a net rate of 6 titles per day, 540 journals added this quarter for a total of over 11,000 journals.