Paul Klee's notebooks are now available online. This is a wonderful resource and if you have not yet read his Pedagogical Sketchbook, can I also remind you that it is mandatory reading for anyone who is serious about drawing practice.
The notebooks of Paul Klee
The Thinking Eye pdf
The Pedagogical Sketchbook pdf
The Thinking Eye and the Pedagogical Sketchbook are seminal texts for anyone thinking about drawing, especially the nature of line and it's attributes. Artist's notebooks and sketchbooks are always of interest, because they are where you can observe the thinking process that lies behind the work. Compare Paul Klee's with Richard Diebenkorn's.
David Zwirner Gallery where you can find more work by Paul Klee