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I think you said you had Steve Roud's excellent book 'Folk Song in England' currently the definitive book on that subject.

For histories and background to English songs you have Steve's and Julia Bishop's New Penguin and similar histories in our 3 volume new editions of the Frank Purslow books in the Marrow Bones series, with a new one on its way next month hopefully.

There are some very good recent books on some of the collectors, Martin Graebe's book on Baring Gould, and a useful book on Charles Marson's life which has a lot of extra info on Sharp and their falling out.

I'm finding loads of reading matter, like Jim, on Academia online. You don't have to join and pay the subs to get most of the stuff. And endless stream of thousands of papers on folksong and related.

Going back to Peter Buchan, William Walker's biography of Peter is very interesting reading of such a controversial character in the early 19th century.

Song collections. Again depends what area, how far back. An absolute must is the 8-volume Grieg-Duncan Collection if you can get it. The Pepys Ballads and Roxburgh are all online at UCSB EBBA.

There's absolute tons of stuff online, more than any of us could read in one lifetime.