The Vaughan Williams Memorial Library at Cecil Sharp House has some of Sharp's field notebooks, and a lot of other material on microfilm; his handwriting has a reputation for being difficult to decipher, though. You should probably try to get hold of a two-volume book called Cecil Sharp's Collection of English Folk Songs, edited by Maud Karpeles (Oxford University Press, London, 1974); this is by no means the full collection, but is the most comprehensive selection available. The material is given as originally noted -so far as I know; I haven't seen a copy yet, though I've just ordered one- not as modified for commercial publication. The book is hard to find, as the original print-run was not large and it's never been re-issued, but Inter-Library Loans or whatever the U.S. equivalent is may turn it up for you.

Issue No.6 of the Journal of the Folk Song Society (first part of Vol.II, 1904/5) was mostly devoted to songs from Somerset and Devon collected by Sharp. The tunes will have been given as noted, but the texts may be incomplete (I don't have a copy); again, libraries may be able to turn it up.