Reddit mentions of The Jepson Manual: Vascular Plants of California

I assume you already have a copy of the Jepson Manual and / or are familiar with eFlora. Sort of the de facto gold standard for California. You can get a used older edition pretty cheap and other than moving a few species around it's still a very usable, if not very portable book.

I'd point out that California has one of the richest floras in the world. In some ways though, it's characterized by a large number of species and subspecies within many of the genera. So if you can learn to recognize for example a plant from the genus eriogonum or arctostaphylos at a glace, keying it out to the specific species will be much more simple. Learn to spot the major genera of the area because the exact species you will encounter might be different than you would see just a few miles away and you will never remember them all, the book is 1600 pages.

Fruit by Stuppy and Kesseler is packed full of gorgeous scanning electron micrographs (and other pictures too) and a lot of very detailed but very readable information. I can not reccomend it highly enough. Seeds and Pollen are also very good. I have not read it (just found it now, going straight on my wishlist) but The Bizzare and Incredible World of Plants, also by Stuppy is almost certainly excellent.

It's a bit technical and dry, but Plant Form, by Adrian Bell is one of my favorite reference books of all time. The information is fascinating, and the diagrams are gorgeous. There's a free online copy available (legal, I think) if you would like to have a look, but I would highly recomend a physical copy, and it's pretty cheap as far as reference books go. Flip through the section on Tree Architecture starting at page 296 for a sample of how cool it is. Read and understand that section and you will be amazed at the things you will start noticing about plants around you.

For plant ID, I can not reccomend Botany in a Day highly enough for a quite comprehensive tutorial in how to recognize plant groups (which makes it orders of magnitude easier to come up with a more specific ID). It's a classic, and is a required text for just about every field botany class.

Getting a good guide to your local plants that is based on dichotomous keys and diagrams rather than photos and learning how to use it is an absolute must if you want to move past the basics for IDing plants in your area. Without knowing your location, it's impossible to give good recomendations, but the Jepson Manual is a good example of what you should be looking for, and by far the best guide to California plants. Unfortunately these sorts of books are usually fairly pricey, and can be pretty impenetrable without practice (helps a lot if you already have a general idea of what it is), so you might hold off on getting one until a much later date. You can get older editions for cheaper, but at least in the case of Jepson's, most of the changes involve more diagrams and easier to use keys, so it might not be worth it.

There are loads of others that are slipping my mind at the moment, I will add them later if I remember.

I asked this question over in /r/marijuanaenthusiasts just the other day.

This Jepson Manual looks like the authoritative text for California, at least. I'm sure with some googling you could find a similar text for your area