I've been studying a little about
the bar construction
. After doing my homework, I'd like to recommend the following sources as good background material on the subject:
Notes by J. Baez (I think) from a seminar titled "Cohomology and Computation"
. The notes are very clear, although hand-written. The bar construction is presented around week 23 (and onwards).
Section VII.6 and around in MacLane's
Categories for the Working Mathematician
Classifying spaces and Fibrations, by Peter May