This paper provides a good concise description of the method:
Rapid Object Detection using a Boosted Cascade of Simple Features. Paul Viola and Michael Jones. CVPR 2001. Paper
This paper is 25 pages long and has more detail:
Robust Real-time Object Detection Paul Viola and Michael Jones. Presented at ICCV 01 workshop on statistical and computational theories of vision. Also submitted to IJCV. Paper.
This paper describes some additional work on a small tweak to AdaBoost to penalize false negatives:
Fast and Robust Classification using Asymmetric AdaBoost and a Detector Cascade. Paul Viola and Michael Jones. NIPS 2001. Paper.