The RFC 2528 is a good document if you're searching for a few litterary explanations of TCP Congestion Concepts, as Slow Start, Congestion Avoidance and Fast Retransmit.
This document defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. In addition, the document specifies how TCP should begin transmission after a relatively long idle period, as well as discussing various acknowledgment generation methods.