I just picked up a book called Unfuck Your Boundaries. It's written by a trauma therapist but the style, if you can't guess from the title, is very casual and easy to read. I am still reading it, so I can't give a full review, but it's approachable and entertaining while covering the fundamentals.

https://microcosmpublishing.com/catalog/books/8188

Children of emotionally distant parents there's also Drama free. My therapist recommended both to me

Not strictly about boundaries, but I think you'd like the Evolving Self by Mihaly Csikszentmihalyi. It goes a lot into how to consider your priorities in life and what you want to spend your time on

It’s slightly academic, but check out Eric Berne’s “Games People Play” about Transactional Analysis theory. There’s a short series on YouTube by a user called TherminTrees (a little cheesy and dated looking) that can give you a quick intro if you’re not sure what it’s all about. Its concepts really helped me.

I bet books for children with narcissistic parents might help. Some of those might not be explicitly religious. Sadly I don't have any recommendations

I don't have any book recommendations but I can't help but feel like the entire approach you're trying to take might be too over generalized and you're better off trying to approach each problematic social encounter one by one.

If for example you have family who's down on their luck and trying to move into your living space despite you not wanting that, you need to consider what their other options are for living alone and if that would result in a quality of life you would be able to accept yourself, and weigh that against your own expectations for how the living situation would pan out in your mind.

Stuff like that family member's previous behaviour, ability to show gratitude and value you equivalently, the degree to which they are responsible for their current living circumstances, etc, are all important to consider. This is nothing generalized advice about "boundaries" could possibly help with imo.

If on the other hand you're a woman and have issues with men hitting on you at work, you have a completely different set of considerations you must make, with virtually no overlap with the previous example.

I have not read them myself, but Nedra Glover Tawaab's books come highly recommended: Set Boundaries, Find Peace + the accompanying workbook, and Drama Free (mentioned here already).

I did a Google Books search of Set Boundaries and only got one hit for the word 'god', very near the end in what looks like the "Commonly Asked Questions" section. Between that and what I see in the previews (looks quite promising), recommend checking this out to see if it's useful for you.

Boundaries are deeply tied to one's worldview, values, and understanding of human nature. If you reject the idea of a higher power or divine authority, you must still grapple with the question of why boundaries are important and how to justify them.

If not God or a higher power, then what is the ultimate source of value and authority for setting boundaries? Is it simply a matter of personal preference, cultural norms, or social contracts?

If you rely solely on human reason, empathy, and self-interest, you will struggle to establish a consistent moral framework for setting boundaries. For example:

1. Why should you prioritize your own needs and desires over others'?
2. How can you justify saying "no" to someone who is making a legitimate request?
3. What is the basis for your moral obligation to respect others' boundaries?

Ultimately, your views on boundaries will depend on how you answer these deeper questions.

In this sense, It seems looking to believe in boundaries without faith (or a deeper philosophical commitment) is indeed doomed to fail, not because faith is required, but because our understanding of boundaries relies on faith and trust.

I know it's not a good answer to the question, but Hopefully my deconstruction helps