If you're not already a regular visitor to the ASIFA-Hollywood blog, you should be. It's a treasure trove of animation and comic art as well as scan of entire books that've fallen out of print, like the original Preston Blair book or Nat Falk's How to Make Animated Cartoons.
One of their current features is high-res scans of Zim's Cartoons & Caricatures, or Making The World Laugh by turn of the (last) century cartoonist Eugene Zimmerman.
What I'm most impressed with is how lively his roughs can be while still being so specific. No vague stick figures or stiff layouts. Plus there's definitely a different style and sensibility to cartooning from 100+ years ago, before the influence of animation.
Apparently the pages are going to start being cycled off soon, so if you want to get the entire book you'd better head over there now. Besides that's the only place you can get it, and it's free.