Scott McCloud's work is outstanding! They were actually my official textbooks in college (I did Comics and Visual Storytelling), and I learned so much from them, and still take them out and flip through them every now and then.
I also found Panel Discussions to be interesting - it focuses less on the physical appearance of the page (though that is discussed as well), and more on the thought that goes into it before you put pen to paper. It's built on interviews with a bunch of big names in mainstream American comics, so it IS very US-centric in terms of work-process - discussing a lot of the intricacies of working with editors and whole teams of artists - which isn't always applicable for us webcomic creators, or comic artists who aren't US-based - but it's definitely interesting, and there is good advice to be had in it.
I haven't touched many other how-to-draw-comics books - I've got a Perspective for Comic Artists-book sitting on my shelf, but that's about it - as I've found it more effective to, you know, just read comics. As long as I read them with a critical and analytical eye, I can learn just as much from an issue of Hellboy as I can from reading an entire book full of how-to's.
