Hi Steven,
Every painter, even if they never paint a landscape, should have a copy of Carlson in the library. Not only does it give the non-landscape painter insight into how very, very difficult it is to paint landscapes well, it is packed with universal principles that govern learning to SEE. Carlson mentions from time to time paintings that are 'stupid'. It seems an odd word, but a description that has stayed with me many years, and can describe portraits as well as landscapes, or anything else for that matter.
The Buechner book...it's gorgeous.
I have also just finished the (just published) Robert Johnson's "On Becoming a Painter" and it is likewise fantastic. It's a great blend of technical and philosophical text, and the color plates will knock your socks off.
Chris
Administrator's Note: This book is not currently available on Amazon or Barnes & Noble. However, it can be ordered from the author's web site at http://www.robertjohnsonart.com/