Universities UK has released Open Access Monographs in the UK: A Data Analysis.
Here's an excerpt:
In 2018, fullstopp GmbH was commissioned to carry out an analysis of open access books. Specifically, we were asked to capture the specific challenges and barriers (perceived and real) from a range of stakeholders including (but not limited to): learned societies and subject associations, Pro-Vice-Chancellors (Research), research librarians, publishers (commercial, new university presses and academic-led presses) and funding organisations. In response to these concerns, we were also asked to (where possible) address these challenges by conducting a quantitative analysis of data available from publishers, funding organisations and HEIs (including libraries).