Anything by Burne Hogarth is excellent. I wish someone had introduced me to his books YEARS ago. It would have made me a much better artist than I am today.
The DC Comic Guide to Inking Comics, by Klaus Jansen is very helpful as well. But his tips can seem a bit biased at times, or almost contradictory. Still worth checking out, even if you're not an inker. (I think it's important to learn EVERY aspect of comic book production, even if you are not actually involved in all aspects).
The most helpful book I’ve EVER read about comics is Dave Sim’s Cerebus Guide to Self Publishing. There is only one chapter in there about illustrating, but there is loads of information in that book which answers questions I never would have thought to ask. It’s kinda hard to find these days, but if you can get ahold of the updated 2011 edition, READ IT.