If a person were to go to any chain bookstore, they would find, next to the few paltry shelves of graphic novels and trade paperbacks, shelf after shelf full to the brim with much smaller, brightly colored (they aren't afraid of pink) books with a variety of bizarre titles. These are called manga.
First, a few definitions, for those who have no idea what a "manga" is. Manga is Japanese comics, and they are very popular in America right now. Very, very popular, with a very vocal fanbase. Saying something negative about Japanese cartooning (which also includes "anime," or Japanese animation) in today's geek culture is strictly taboo. Anime/manga conventions are to modern geek culture what comic book conventions were in the 80s and 90s and sci-fi conventions in earlier decades; they are nexus of geek culture.
So, why are these things so popular?
That question is beyond the scope of this article, but these comics differ from traditional Western comics in a variety of ways, both in format and content. If you were to pick up one of these bright, shiny little books, you'd notice right off, despite the colorful cover, they are often purely black and white inside. Now, try to read it. You'll probably find it very hard to follow at first, even if you are aware that manga, like Japanese writing, reads right to left, instead of left to right (in case the back cover being on the front didn't clue you in). First time readers of manga will often find this distracting.
First time readers of manga will find a lot distracting. While Western comic books prefer to stick to one artstyle throughout, manga often makes drastic changes to artstyle and level of detail in order to show emotion; this may sound confusing, but it works much better drawn out than it sounds.
Of course, being Japanese, these books were originally written in Japanese. Many American editions go out of their way to stay faithful to the original (though, of course, there are occasionally just simply bad translations); many include a "translation guide" to explain idioms or even puns that just couldn't be translated. However, most sound effects are written in Japanese characters; these are retained in most translations, so the page is often scattered with them, sometimes with accompanying translation, sometimes not. If you know Japanese, getting the original is probably a good idea; however, this guide does not recommend you learn a new language just to read comic books that are readily available in English, anyway.
Content wise, manga, to many people here in the West, is just plain weird. Manga, historically, owes more to Western animation than Western comic books; Japanese cartoonists basically learned everything they could from old American cartoons, then brought their own unique cultural vision to the equation, which is very different from anything in the West. Much like the constantly shifting artstyle, the very mood of manga changes drastically; just as manga cover artists aren't afraid of pink, manga writer/artists (usually a manga is the product of one writer/artist, more akin to Western "underground" comics) aren't afraid of being cute, even in a work that is otherwise best described as horror. Readers new to this style may find it jarring.
Even more jarring is the explicit, almost fetishistic, sexuality of manga; manga artists are not afraid to show a little skin, and full frontal nudity is not uncommon in any genre, though, unless explicitly adults only, most manga characters have the apparent anatomy of your average Barbie or Ken doll. This comes from a culture that, unlike the West, does not give as much importance to the "dangers" of imagination, professing a "it's not real, so it can't hurt" philosophy while, at the same time, enforcing much harsher censorship laws. In other words, Japan is both more and less repressed than America, and this paradox is expressed most keenly in Japan's cartooning.
Perhaps this guide does have an explanation for the popularity of manga in America.