"Otaku" is one of those Japanese words that has no precise equivalent in English. "Geek" translates the knowledgeability as well as the social ineptness of the stereotypical otaku, but not quite his (and, more rarely, her) intense interest in what so-called serious adults regard as trivial pursuits: Anime, manga, games and other Japanese pop culture manifestations.
In any case, the word has been enthusiastically adopted by foreign fans despite its negative connotations in Japanese, which include loner, loser and homicidal maniac — as in the notorious "Otaku Murderer," Tsutomu Miyazaki.
Books targeting these overseas otaku have become a thriving publishing niche, with the typical aim being to supply facts about the objects of obsession, while opening windows into the mysterious culture that produced them.