Whither Sketching?

I've had a lifelong interest in methods for increasing attention. Note-taking, annotation, highlighting, are all excellent methods for improving retention of information and also clearing out the clutter. Sketching is another method for distilling a scene or experience down to its essence. Regrettably, I've had little luck finding a book that deals with true sketching, i.e. a quick way to draw a subject without getting caught up in detail and realism. I'd appreciate suggestion for good books on the topic. I have book on the topic of field sketching on the way from Amazon.

The Metropolitan Museum of Art is doing an exhibit of Da Vinci's sketches thru March 2003.