The art instructor that I think I have learned a lot from was watching Joseph Yarnell (Jerry) on PBS for many years.  I have all of his books and still look at some of the old tapes I have of his work.  He explains things that they do not tell you in art school.  I went to one of his workshops which was on composition and perspective and was very interesting.  His website is YarnellArt.com. 

I have had several art teachers and instructors over the years since I have been painting.