I want to start my reviews and recommendations off with what I think is the most important book regarding comics ever published: Understanding Comics by Scott McCloud.
Although I remain a huge fan of superhero and adventure comics, I'll be the first to nod my head and pump my fist when McCloud starts Understanding Comics off by separating content from form. In other words, just like any art form (novels, poetry, film, painting, etc), comics can be about anything and shouldn't be dismissed just because the most well-known comics so far have been people in goofy costumes beating each other up. McCloud, a comics writer and artist, has produced a book (in comics form) that completely breaks down what comics are and how they work. And if that sounds boring and intellectual, let me tell you, he does it with loads of charm, humor, excitement and wonder. He makes thinking deeply about comics fun and interesting.
I can't recommend this book highly enough. If you love comics, this will deepen your love. If you're one of those people who has a hard time getting into comics, this will help you understand why you can't go into comics expecting them to be "illustrated novels." (You may still not like comics, and that's fine. I'm not really big on ballet. But at least you'll understand the medium better.) And if anyone tells you comics are just silly cartoons or are inferior to novels, give them Understanding Comics to read.