For those who are interested in Klee's personal notebooks, this collection of Bauhaus eBooks on the same site could also be of interest (iirc, HN brought me to these a couple of months ago):
http://monoskop.org/Bauhaus#Books
Klee is proof for the 'practice minded' set. Certainly he had natural artistic talent (and was an accomplished chamber musician), but he worked very hard to arrive at a fundamental understanding of color.