Robert Utley put together a small book in 1989 called The Department of Everything Else. The full text is online at the link. It's a short, readable book -- I suspect you'd be able to get through the entire thing in 20 minutes -- but as I recall there's nothing really earth shattering there.

As the title suggests, Utley claims that the Interior Department's extremely odd set of responsibilities today comes from its history as a clearing house for miscellaneous government functions. Some of those functions were spun off into other agencies, while others were added to Interior's portfolio. Utley represents the process as fairly slapdash. But if I'm missing some grander narrative I'd be interested to hear it from others.