There are several platforms through which you can publish OER textbooks.
Pressbooks is the platform being used by the CAUL Open Educational Resources Collective and other Australian university OER publishers (e.g. Western Open Texts from Western Sydney University and RMIT University Open Press). Pressbooks is an online book content management system that allows authors to design and publish online books in a variety of formats. They provide a number of pre-formatted book templates which make the authoring and creation experience easier. Use the links below for access and support for Pressbooks: