Can be fiction or non-fiction, but I have in mind non-fiction books on topics so obscure you'd be the only person who'd want to read it in the first place.
My two are:
1) A book about the relationship between journalism and superheroes. Unfortunately, my two case studies are Clark Kent/Superman, and Peter Parker/Spiderman, and I think I would need more than this to build a case on. Still, I reckon it would be awesome.
2) A book about the lyrics of Conor Oberst - like Christopher Ricks did for Bob Dylan. It would be really pretentious, with chapters like: When the President Talks to God - Prayer and Politics in the Later Albums, and From False Advertising to Light Pollution - Oberst and the Sickness of Modernity.