Dicho, the Kennedy book (A Treasury is an enlarged hardcover) is, in my opinion, a "hotchpotch" collection (398 pages) of poems, old popular songs, folk songs and so on (not a book of ballads in the strict sense of the word). I don't think it's a must; however, there're a lot of less-known poems and songs in it. Texts only, and little info on background.

~Masato