Understand the editorial principles that make scholarly books reliable sources of information.
Many scholarly books have a very limited readership, consisting of scholars, teachers and students studying a particular topic. This makes them unsuitable for mass market publishers. University presses arose to fill this niche of publishing book length academic research.
Dr. Leonard Allen of Abilene Christian University Press spoke in 2012 about the mission of university presses.
Open access books are freely available to read online.
Instead of funding the production of the book by selling physical copies, other types of financial support are required. For example, the publisher may ask authors to pay a publication fee. Authors may in turn ask their institution to cover the fee or apply for a research grant from a government or other funding agency.
This short excerpt from a video by a major publisher, Springer Nature, describes their open access publishing initiative involving book processing charges (BPCs).