It’s always great to be able to support authors by buying their books, but sometimes the cost of self-education can prohibitive. It is my firm belief that any of the authors listed below (unless they’re dicks) would not want anyone to be prohibited from reading their work. This is why sources like www.libgen.is and www.sci-hub.se and other book sharing websites exist. In this case I suggest purchasing a cheap android tablet and becoming acquainted with the idea of reading textbooks in pdf form. A half pound electronic device can store upwards of half a million pages or more worth of textbooks.
Phylogeny and Evolution of the Angiosperms by Pam and Doug Soltis
The Tangled Tree by David QUammen - A Good pop-sci explanation of molecular phylogenetics and understanding evolution
Plant Evolution : An Introduction to the History of Life by Karl Niklas
Botany Illustrated by Janice Glimm-Lacy
Annals of the Former World (geology) by John Macphee
Botany for Gardeners by Brian Capon
The Ecology of Plants by Jessica Gurevitch
A Botanist’s Vocabulary by Susan Pell
The Rose’s Kiss by Peter Bernhardt
Flowering Plant Families by Wendy Zomlefer
How the Earth Turned Green by Joseph Armstrong
The Origin, Expansion, and Demise of Plant Species by Donald Levin
The Ecology of Seeds by Michael Fenner
The Fungi by Sarah Watkinson
Biogeography : An Ecological and Evolutionary Approach by C. Barry Cox
Evolution Making Sense of Life by Carl Zimmer
Cacti Biology and Uses
An Island Called California by Elna Baker (a great text due to it’s explanation of ecological relationships even if you don’t live in California)
Serpentine Geoecology of Western North America by Earl Alexander
A Natural History of California by Allan Schoenherr
Ecology of Desert Systems by Whitford
The California Deserts by Bruce Pavlik
Plant and Animal Endemism in California by Susan Harrison (again - great explanations even if you’re not into California. IT is a great case study)