One of the more challenging folk books out there is Fakesong by Dave Harker. It deserves careful reading and consideration, but ultimately it deconstructs folk song as a category and leaves us without our favorite genre! It takes potshots at all the sacred cows of English folksong scholarship, and takes everyone from Sharp to Lloyd down a peg.
Alan lomax's The land Where the Blues Began is a beautifully written book.
for sea songs, look to Bill Doerflinger and Gale Huntingdon as well as Hugill.
Kenny Goldstein and Edith Fowke's book on Bawdy Ballads and Dirty Ditties should be coming out soon, if it's not out already; Rochelle Goldstein told me they had a publisher almost two years ago.
Like everyone else, I have too many favorite folk books to pick from--but how about Banjo Patterson's Old Bush Songs; Kenneth Peacock's set of Newfoundland and labrador ballads (soon to be out on CD-ROM); and Anthologie de la Chanson Francaise: La Tradition. So many great songs...