As far as grabbing the bull by the horns goes, Harley Brown's book,
"Eternal Truths for Every Artist" is a must for oil portraiture and oil painting in general.
It is very motivational, informally written, and makes you willing to pick up the brush no matter what your state of mind.
As for painting and drawing overall goes, I cannot recommend Anthony Ryder's
"The Artist's Complete Guide To Figure Drawing" enough. Superbly written, with absolutely beautiful examples.