I've just recently finished what is, I believe, the textbook that must be read by all true manga fanatics. I speak of course of Frederik L. Schodt's Manga! Manga!: The World of Japanese Comics.
Oh sure, it's dated, but holy crap is it ever good. I've seen series and information about series I might never have heard of had I not read this book. Such wonderful information about the history of the artform, which also reflects the history of the Japanese nation, warts and all.
To my fellow forumites, I highly reccomend this book since Mr. Tezuka, Schodt's friend, wrote an introduction to book and seemed to predict the eventual succcess of manga in the states.
Furthermore, there are four samples of some of the best in manga. There's a sample of Osamu Tezuka's Phoenix, a short story from the Senjo Manga war comics series by Leiji Matsumoto, a sample of Rose of Versailles by Riyoko Ikeda, and a sample of the near-perfect anti-war manga Barefoot Gen by Keiji Nakizawa, all in that order.
It's a must read, in every since of that phrase.