It's been a while since I've read it, but I remember The E-Myth Revisited by Michael Gerber being a very interesting take on how to build a scalable business by analyzing how successful franchises operate. It was an unexpectedly practical and engaging book, even for someone not interested in franchising per se.
I don't know if you could call it non-mainstream but buying "Pocket Reference 4th Edition" has been a source of joy for me. Pocket sized "wikipedia". It contains everything from how to tie knots to the specs of sheet metal screws.
For example — how you build a feedback loop and why it's more important than writing elaborate guides, docs and business processes.
Or how you give a simple yet meaningful metrics to your tests and learning process.