Horkheimer page on cmu critcal theory web
Max Horkheimer and the Philosophy of Education useful page though not the most insightful analysis.
The Horkheimer Pollock Archive (where Horkheimer's stuff is stored . The place to locate all the letters & papers of MH relevant to serious research)
Origin Myths in the Social Sciences: Fromm, the Frankfurt School and the Emergence of Critical Theory An essay which discusses Fromm's influence on early critical theory as a member of the Frankfurt School in the late 20's and early 30's, and his subsequent disappearance from the"official" history of the Frankfurt School after his break with the Institute for Social Research. Fromm was a major influence on Horkhimer's early work.
Michale Wenz, Personality Authority, Society, This is an essay which analyzes some of the contributions of Horkeimer and his collegues to the study of personality focusing on Studies in Prejudice and the Authoritarian Personality.
If anyone reads german this is a study of Horkhierms early philophical development as a studnet of the neo-Kantian Hans Cornelius thorugh to what the author terms an historical materialist phenomenology. I don't know know of any comparable study in of his early work in English.