Barrett is used as a source of several songs in "The Painful Plough: a portrait of the agricultural labourer in the nineteenth century from folk songs and ballads and contemporary accounts" by Roy Palmer, not an easy book to find in itself, but easier than Barrett's own book. Songs from Barrett in "The Painful Plough" are The Sheep shearing, The painful plough, and The honest ploughman. I've been able to borrow a copy of Barrett's "English Folk-Songs" from my local library through WorldCat (that's a world-wide library catalog searchable at worldcat.org). You just have to pay postage for the book when it arrives. I'm still waiting excitedly for it to come.