There are precious few books around that really treat the comics medium seriously, or fully explore what is and may be possible in that medium. Scott McCloud's "Understanding Comics" is one of the few and - for me - is the best of the bunch.
This book has tonnes of things going for it: McCloud's enjoyable and accessible cartoon-style delivery, which is itself a brilliant demonstration of how powerful a communication tool comics can be ... the infectious passion he has for his subject ... the bountiful results of his serious research and scrutinization ... I could go on.
Perhaps the best thing about "Understanding Comics", though, is McCloud's sheer imagination. When it comes to comics, he has a way of thinking and seeing that is almost completely uninhibited by any preconceptions. The result: he's constantly chucking ideas at you that surprise you and make you re-think about comics, writing, art and perception. You might not agree with everything he says, but the point is you're thinking about something you'd never have considered before. It's a truly eye-opening piece of work.
For readers and writers of comics alike, "Understanding Comics" is invaluable in helping you to appreciate 'sequential art'. For those who have never considered comics worth bothering about ... I urge you to get this book and read it. I guarantee it will make you think again, about comics in particular and art in general.
One final note: as I'm sure Scott McCloud himself would say, don't stop here. Check out other works such as "Comics and Sequential Art" by Will Eisner, or Alan Moore's essay "Writing for Comics". And any sequential art you can lay your hands on!