I checked out a really interesting book on the Northern Cities Vowel Shift a couple of days ago from the library called Small-Town Values and Big-City Vowels: A Study of the Northern Cities Shift in Michigan. This book, detailing a study done by Matthew J. Gordon on two Michigan communities, was published by the American Dialect Society in 2001 so the information is pretty recent. If you're interested in dialectology and specifically how English is changing in the Northern Midwest in the US, check this book out, as there's a lot of interesting information there. I just finished reading the detailed historical introduction to the shift, and as I suspected it's not as straightforward or homogenous a shift as some might think.
Also, reading about the specific details of the shift, I was amazed at the parallels I could draw in terms of the California Shift (in terms of the drag-chain theory especially) in theory, altho of course in actual practice the two are moving in completely opposite directions. Reading this study makes me wanna just pack up and go and listen to people talk in the Northern Midwest--hehe. But seriously, if you have access to this book and are interested in this kind of thing, I'd highly recommend it.