I would like to recommend a book, which has been recommended before me by Daisuke Tsutsumi. Alla Prima by Richard Schmid, http://www.richardschmid.com/ It is a must have book for anyone serious about painting. Full of Ah-Ha! moment and revelations, a cheap price to pay for the real value in it. The focus is not on technics, but more on various principles. Relevant to any medium you might choose.