Consider your options: explore the different funding streams for Diamond Open Access
This guide (an output from the DIAMAS project) lists different funding streams for Diamond OA publishing and service provision. While it is mostly focussed on journals, there are elements of it relevant to books. It goes through a range of models, some of which are only relevant to journals (e.g. subscriptions to print copies) and some of which are more broadly applicable (e.g. collective supporter programmes), describes each model, sets out who the funder is and what exactly is funded, and provides examples. It gives pros and cons for each model. In our opinion, the pros and cons provided are realistic and reasonable.