Instructions exist on /BuyingMangaFromAmazonJP. Are there any good single-book Japanese comics available in English for those who would like to do a little research and compare and contrast with American comics, without having to get into an N+1-issue ongoing series which will eat up arbitrary amounts of money and shelf space? Something like From Hell or Ghost World rather than Hellblazer or even Transmetropolitan.
Try your local ForbiddenPlanet. They'll have /KazeNoTaniNoNausicaa?, which is four volumes (the anime covers the plot from just the first volume) and rather excellent. They'll also have the /RumicTheatre? series if you're after light-hearted stuff (short stories, 3-4 per volume, all unrelated - just get any one volume) and /Rayearth? (3 volumes; don't bother with the sequel) if you're after really pretty artwork. If you're after something really good, short, angsty and not particularly cutesy, try /Clover (two volumes, self-contained; volumes 3 and 4 are sequel and prequel respectively, only get them if you liked the first two; only available in Japanese, unless (ISBN 1892213664 , ISBN 189221394X ) are in English - I see nothing on the page to suggest it; but there are scripts you can read along to around) Heck, you can even borrow our copies of any of these if you're in Cambridge.. ;) - MoonShadow
Can't swear to it but the two volumes of Clover linked above should be English. It certainly exists in that form. The others on that page are also quite interesting although putting the cover to Wish on Angelic Layer is a bit worrying... - Kazuhiko
Speaking of which, /Wish? is another pretty good one, and only four volumes. Rather more quirky than most of the ones suggested above, though. I also strongly recommend the AlienNine? manga (the third and final volume of which comes out in September), but be advised that it's about disturbing elementary school girls (ambiguity intentional).
Also deserving of mention are the classics, of course. Akira? (six volumes) and GhostInTheShell (one volume, though there's a very different sequel coming out at the moment), both much better than the movies based on them.
And finally, supposedly the Hellsing manga will be coming out from DarkHorse? soon too, but that doesn't fit the requirement of being short....