Diamond Open Access (OA) also referred to as platinum OA is a model of publishing which does not charge a fee to the author or the reader. Typically scholar-led and community owned publishing models with the sole purpose of making research OA for the benefit of society and not for profit! This model offers an alternative to the BPC and is seen as a more sustainable and equitable route to achieve OA for both journals and for books.
Libraries are supporting Diamond OA for books via consortiums or memberships where they pledge funds to make monographs OA retrospectively (such as with Knowledge Unlatched), or pay a membership to the publisher that covers the cost of OA (e.g. Open Book Publishers). In this way, libraries are supporting Open infrastructure and helping more authors to publish their work OA, not just at their own institutions but also those authors who may not have access to funds to make their work openly accessible.