Along with Rainbow Quest, I would recommend these books: Romancing the Folk: Public Memory & American Roots Music by Benjamin Filene; When We Were Good: The Folk Revival by Robert Cantwell, and Ron Cohen's more recent Folk Music: The Basics. For a more academic perspective, look for Transforming Traditions: Folk Music Revivals Examined edited by Neil Rosenberg. And if you can find it, the 1967 Dell paperback, The American Folk Scene: Dimensions of the Folksong Revival contains a lot of still-essential esssays.

Dick Weissman's book contains a lot of good information, but I think it suffers from tunnel vision as it describes folk music as a business, and mostly ignores the social phenomenon that is an important part of the modern revival.

I'd also recommend several books about specific people and/or scenes: Baby Let Me Follow You Down about Cambridge and Boston in the Sixties, Positively 4th Street about Bob Dylan, Richard Farina and the Baez sisters, Dave van Ronk's The Mayor of MacDougal Street (with Elijah Wald), and Bob Dylan's Chronicles, among others.

For the transition from the folk music revival of the sixties into folk rock and beyond, Ritchie Unterberger's two volume history of folk rock is essential: Turn! Turn! Turn! and Eight Miles High.

Add two books about important folk music record labels and how they both fed and were norished by the folk music revival: Jac Holtzman's Follow the Music about Elektra, and Making People's Music: Moe Asch and Folkways Records.

Gary Cristall's history of folk music in Canada is a work in progress, but it's useful, considering the huge Canadian influence on the American revival: http://folkmusichistory.com/intro.shtml.

As for the revial in England and the UK, look for The British Folk Revival: 1944-2002 by Michael Brocken. It's not the definitve history of BritFolk, but it's a useful introduction.