Man, that's a loaded question.
Mystery book series, people! :D
For me it was Ed McBain's 87th Precinct novels. I got into this hard R-rated crime series at the ripe old age of eight. I still remember standing at the library check-out desk lifting high a stack of these hardcovers to a very uncomfortable librarian who stared at my mother asking, "Um, are you sure about this?" And receiving a cold unflinching High Noon-worthy thin-lipped nod. Probably explains a lot about how I turned out.