For open monographs, collective library subscription is the key

"So even if you buy the argument that taxpayer-funded research should be free to all, is it ethical to force monograph publishers – frequently mission-driven and not-for-profit – to undertake much of their work, in essence, for free? Or should green open access for books use preprints that are produced prior to any publisher labour?...

A better model for open monographs is collective subscription. Already successfully implemented by a range of university presses, this involves libraries repurposing their existing collections budgets to support a transition away from the default gated access model. These initiatives do not demand additional funding or encourage a two-tier division of institutions and disciplines. They don’t risk humanities authors being short-changed or undervalue the publishers most committed to long-form scholarship...."