"Otaku" in the west refers to a fan of Japanese anime (cartoons) and manga (comic books). But here it refers to obsessive fans of anything - computers, trains, J-Pop idol groups, cars, baseball...
I would consider myself a baseball, amusement ride, and photography otaku. I am not a camera otaku; I can't tell you anything about camera models - I usually can't remember even which brand I have - but I love the action of photography and later sharing my work on my other blog.
Akihabara has pretty much everything that fits the otaku label, but especially anime/manga and idol fans.
I saw the books a couple years ago at full price, but I found them on clearance so I picked up one of each. There's no way I'm buying a dozen of each book to build a set, though.