Since you do follow Language Log, I wonder if you are thinking of tackling "The Cambridge Grammar of the English Language".:D
Haha! I've thought seriously about it, because of how very much I enjoy Geoff Pullum's writing, but it would be a BIG challenge. If I do the fiftyfifty challenge again next year, I will be starting in January, not April, so maybe I will be able to nibble away at the CGEL over the year. If there's an e-book version then the odds of my trying will go up. :)
BTW, have you watched: Supermen of Malegaon. If non-fiction books count towards the 50-50, I guess documentaries should too.www.youtube.com/movie/supermen-of-malegaon
It's high on my list, definitely sometime in the next week.