What are some less-known spiritual books you recommend?

Any religious or spiritual tradition recommendations are welcome! I have a growing collection of Catholic literature, and I am interested in exploring more Christian mystic, Sufi, Buddhist, and Taoist writings in particular.

I'm especially interested in religious and spiritual classics/contemporary reads that may have flown under the radar when they were first released, or that aren't as commonly discussed in spiritual spaces. I'm already familiar with the works of St. Augustine, Thich Nhat Hanh, and Alan Watts, and I want to branch out beyond the big writers.

 

I don't know hardly anything about Alan Watts, but I do remember looking for 'Mort Sahl' mentions, and read (and maybe you have, too) that Alan moved to SF in the early 50s, and would see Mort at the hungry i, when Sahl invented a new kind of humor.