After reading Griffiths-Morgan, Bott-Tu (not just the chapter on Rational Homotopy Theory, I would say) and Felix-Halperin-Thomas, maybe it wouldn't be a bad idea to be acquainted with:

1. Halperin, Lectures on minimal models, Mémoires SMF 230 -aka "the bible": all technical details you won't find elsewhere.
2. Bousfield, Gugenheim, On PL De Rham theory and rational homotopy type, Memoirs AMS 179 -the model category point of view; Sullivan's results can be stated as an equivalence of categories: find which.
3. Lehman, Théorie homotopique des formes différentielles, Asterisque 45 -if you know French, this is a very nice introduction to the subject.