From Wikipedia:

Open Book Publishers (OBP) is an open access academic book publisher based in the United Kingdom. It is a non-profit social enterprise and community interest company (CIC) that promotes open access for full academic monographs, critical editions and textbooks in the Humanities, Social Sciences, Mathematics and Science. All OBP books are peer-reviewed.[1][2]

All OBP titles are available in free, open access editions in PDF and XTML formats on the publisher’s website, and a number of platforms including Google Books, Worldreader, OpenEdition, DOAB, The European Library and Europeana.[3] Some editions are hosted on Wikiversity in socially editable format (e.g. In the Lands of the Romanovs: An Annotated Bibliography).[4] Readers in developing countries can access OBP titles using e-readers and 2G mobile phones via Worldreader.