Eelco Ferwerda has published "Open Access Monograph Business Models" in Insights: The UKSG journal.
Here's an excerpt:
In recent years, a number of business models have been developed for open access (OA) monographs in the humanities and social sciences (HSS). While each model has been created in response to specific circumstances and needs, some commonalities can be observed. This article outlines some of the main types of model to support the costs of publishing OA books and provides examples of these models across the world.
It is followed by three short sketches providing more depth on: firstly, a traditional publisher's OA monograph offer; secondly, a licensing-based model which draws from existing library budgets; and finally, an experiment with delayed open access for books in philosophy: http://dx.doi.org/10.1629/2048-7754.118.