Hi Michele,
In addition to Carlson and Payne, you may want to check out the new
Plein Air Magazine . I haven't had the chance to read this magazine but I'm looking forward to subscribing to it.
I often refer to
Richard Schmid Paints Landscapes
which I hope will someday be reprinted (with more color plates) so that it won't cost so much to buy. (Inexplicably, I found it at a used book store for $15 a few years ago.)
You might also pick up a (much less expensive, Dover edition) of [
The Artistic Anatomy of Trees