As with most publishing houses, there’s usually a large backlog of texts and books and manuscripts that are sent in and many of these publishing houses suggest getting agent representation or checking on their sites for when they do call-outs for manuscripts on certain topics (Penguin Books is one such publishing house that has a guide as to how to best submit on what and when).
In terms of resources in general, there’s a few publishers that exist (and this is by no means an exhaustive or recommended list):
- Scribe Books – an Australian group
- Prometheus Books
- Atheist Republic
- Freedom From Religion Foundation
- The Oak Hill Free Press
- Oxford University Press (and any other university press that may publish similar works)
- There is also self-publishing groups like Lulu.com
You can also look through books that are on a similar topic / theme to yours in bookstores and contact their publishers. In addition, Catherine Deveny published a writing guide book in 2016 that may be worth consulting; there are many other authors with tips and ideas that you could consult.