I found the following book very interesting:

'Folk Song: Tradition, Revival, and Re-Creation' ed. Ian Russell and David Atkinson, The Elphinstone Institute, University of Aberdeen, 2004.

This is a collection of papers given at a conference to mark the centenary of the Folk-Song Society in 1998. It sounds a bit dry, and it is a bit of a mixed bunch, but it kept me happy for a couple of weeks. There are some very interesting and provocative papers here.

You might also like to look at a song book published by the EFDSS:

'Still Growing: English Traditional Songs and Singers from the Cecil Sharp Collection', ed. Steve Roud, Eddie Upton and Malcolm Taylor, 2003. The introduction to this book, by Vic Gammon, is an extremely well written, authorative and insightful overview of the career of the great English folk song collector, Cecil Sharp - unmissable!