I did adore Chaos 3.5. What a labour of love that was, especially coming off the back of a pretty dreary era in 3rd Ed codices where there was barely any content at all. I recently read back through the 3rd Ed Eldar Codex, and apart from a couple of nice art pieces and some fun in-universe content, it's a sad, sad thing. 2nd Ed Eldar was lovely, still have that fraying in a cupboard somewhere.
My brother's owned loads of codices and army books, I've only ever had a few: Eldar (2nd, 3rd, 6th), Space Marines (5th) and Tomb Kings (8th). Now I've an iPad, I'm slightly tempted to acquire more!