I recently finished reading the fifth volume of the Babylon 5 mirror universe by Gareth Williams.
Bloody hell, that's a name and title I haven't heard in a long time...
It's definitely the best fan fiction I've ever read. I tried getting into it several years ago (only getting through volume one), but that was before I had an e-reader. It's hard to slog through over a million words when staring at a laptop.
You can get the book for free on his website. I got it in text format and then used Calibre to turn it into Mobi/Kindle format.