Originally Posted by Yorky
Depends in the medium, but I agree Youtube is an invaluable source. There's no substitute for getting out there and doing it.
Any good book on composition, perception, general painting technique in your medium should do. Plein air only differs from studio really in having to consider the difference between the light on your painting outdoors and when viewed in different conditions indoors, and also the impact of outdoor weather conditions on your medium (heat, moisture.) The rest is just practice!
If a book of any kind gives you inspiration, then that's your book.