Here's an excerpt from the announcement:
Between June 2011 and November 2012, fifty Open Access monographs in various subject areas were published in Open Access by nine participating publishers. For every Open Access title, the publishers provided a similar title that was published in the conventional way. Data were collected about usage, sales and costs, to study the effect of Open Access on monographs. OAPEN-NL consisted of a quantitative and a qualitative research component, measuring the effects of Open Access publishing and the perceptions and expectations of publishers and authors.