I would recommend against starting with Astroboy. While it is by my all time favorite manga author (Osamu Tezuka), he has done much, much better work. As a starting point for him, I reccomend these:
Black Jack- It's about a docter named Black Jack who is the best surgeon in the world but doesn't have a liscence to practice. He lives alone on some island taking the cases of those willing to pay his exorbitant fees. Only the most desperate and most bizzare are willing to seek him out. This is one of the most original manga I have ever read.
Pheonix: A Tale of the Furture- This is quite possibly the greatest graphic novel ever published. It a philosophical work in a 60's sci-fi setting. In its 250 pages, it covers more than other entire series, and does it at a leasurely pace.
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