The bottom two shelves of the middle bookcase houses my roleplaying games. Dungeons & Dragons books, adventures, playing aids, dice, etc. I mainly played D&D but I was also into Gamma World, d20 Modern, and Call of Cthulhu. Yeah, I had a lot of interests as a kid (and continues as an "adult kid"). I was a jock, musician, and geek (in no particular order) all rolled into one.
On the far right bookcase, I have my musical equipment manuals as well as music software, music theory and training books, etc.
On top of everything is my National Geographic magazine collection.