From the US end of things, if you're interested in the New York and Boston folk scene of the 1960s, Bob Dylan and Joan Baez, and the somewhat under-remembered Richard and Mimi Farina, I would highly recommend David Hajdu's book "Positively Fourth Street". For an encyclopedia-type book, the incredibly comprehensive thousand-page folk volume in the Music Hound "Essential Album Guide" series is, indeed, essential.