The permaculture books that I refer to most are
Gaia's Garden by Toby Hemenwayand
The Farmer's Handbook.
I like
Gaia's Garden for the overall design ideas and overarching themes. The Farmer's Handbook is awesome for permaculture tricks, like making leaf pots and air nurseries and really understandable instructions for grafting, propagating plants, managing a
compost pile, and making liquid manure. Since the Farmer's Handbook was written for farmers in the Himalayans, not everything applies to my climate, but everything is aimed for doing without spending much or any money. I love that, as I don't have much money!
Aside from those specifically permaculture books, I refer to
Mini Farming: Self-Sufficiency on 1/4 Acre Paperback by Brett L. Markham a lot for knoweldge about amendments and mineral deficiencies, as well as growing tips (he's not particuarly Permaculture, but focuses more on Square Foot
Gardening techniques). I also refer to
webpage for growing tips and planting schedules specific to my region.