These are the only three books I ever found useful!
https://www.abebooks.com/9780004125237/Alwyn-Crawshaws-Watercolour-Painting-Course-0004125231/plp
https://www.abebooks.com/9780823029938/Making-Color-Sing-Practical-Lessons-082302993X/plp
https://www.abebooks.com/9780395205488/Nicolaides-Natural-Way-Draw-Working-0395205484/plp
I found that the most important thing of all was to learn how to draw from life with a pencil. If you can do that, watercolor really just becomes your chosen medium. I always gravitated towards watercolor because of its portability. It is definitiely not the easiest medium, in fact it is probably the most difficult because you really have limited options to correct things. Once the water dries, you can "kind of" fix things but it's certainly not like oil where you can scrub and redo big areas.
The other big thing I learned after a long time is to basically go in with really strong values. So much watercolor I see people really "pussyfoot around" with really weak values and it shows up in the painting.
Don't use a tube black! Mix your own black (I have a secret recipe) and use that as your base. After a while you can figure out how to modify your homemade black to be any color you want. The stuff that comes out of a tube is always very weak.
Be aggressive and don't be afraid to fail. If you are painting to impress other people you might be at it a while before you actually come up with something that can go in a frame.
The greatest watercolor painter of all was Andrew Wyeth. You can learn a lot about watercolor just by studying his work. Also worth looking at are John Singer Sargent and Winslow Homer. Sargent's color schemes in particular are hard to fathom. Homer managed to capture both Bermuda and the backwoods of Maine with equal ease and that is a difficult thing to do.
