This book* by Ruth Perrin is an absolute marvel of qualitative research in the complicated theological and cultural context that is millennials. She interviewed 47 young people from across 23 Protestant denominations, all living in Newcastle upon Tyne, the Tees Valley or Northumberland, and who had all been actively-affirming Christians in their early 20's - now aged 29-37, she explores a rich and deep and thoroughly challenging set of ways in which their faith has been knocked, changed, reformed, dropped, and so on. I've probably read this more like two or three times because it spoke so deftly into the particular kinds of things I've seen young Christians wrestle with (often, as she discusses, largely without helpful support from church families, for a number of reasons) and the fallouts of these issues; though not wishing to simply offload an attempt at a personal summary of her whole arguments and methodologies (fascinating though these are), particularly because she has a fuller book about the same thing coming out soon and I'd rather discuss it in detail then (as I will definitely be getting a copy when it comes out in January). Not much point making a recommendation to you, dear audience, when there is very little chance you would be able to acquire a copy of this particular book either - but it was a highly informative and illuminating exploration of the field and would probably surprise most readers as to the fluidity of religious faith and some of the dynamics underlying this.
* No link on this one, sorry! I know it's against my own rules but hey, it was a limited run exclusively shipped to church research folks and I can't find a page for or about it online anywhere. Check out her blog Discipleship Research though. And consider pre-ordering her next wider-audience book which will probably build on a lot of the research discussed in this.