My dd (16.5 months) tends to have serious favorite books, too. Usually, a book retains the exalted status for about 3 days and then returns to high rotation, but not top of the list. To keep from going crazy reading Aurora: A Tale of the Northern Lights fifty times in a row, I came up with this rule: we must alternate books. So, I'll read Aurora, but, then we have to read a different book before we read the favorite again. This seems to work because she can understand the concept of one in between, but still gets her repetition.
Maybe that would help? I still read the same books a million times, but I do get a break.