Fantasy epics. For the series I've read, I can tell you almost every plot twist, list almost every character and deconstruct why they did what they did, the reasons behind it and what it's symbolic of. Even for those books I haven't read in years, with minimal prompting, I can go over almost obscure events and chat about them. The series I know best are:

A Song of Ice and Fire - George R. R. Martin
The Belgariad/Mallorean/Elenium/Tamuli - David Eddings
The Malazan Book of the Fallen - Steven Erikson

and I have a passing familiarity with the Wheel of Time series, the Saga of Recluce, Lord of the Rings, all the Thomas Covenant series and all of Terry Brook's Shannara books.

I nerd out pretty hard over fantasy stuff, but it's a guilty pleasure of mine.