GENERAL WORKS
Much of the secondary work on the Frankfurt School especially that which was published in the 1970's and early 80's is -- how should I put it politely -- crap. There is no real point in reading the worst of it unless you are either a masochist or are writing a PhD thesis --which amounts to the same thing. What follows then is a selctive listing of works on all members of the School or on general aspects of critical theory
The Dialectical Imagination : A History...
The Frankfurt School : Its
History,...
Introduction
to Critical Theory :... 
Between
the Norm and the Exception : The...
Some Other Books on the Frankfurt School
Richard Wolin The Terms of Cultural Criticism : The...
Ben Agger, The Discourse of Domination: From the...
Peter Ewe Hohendahl, Reappraisals : Shifting Alignments in...