Congestion control is, often, implemented at the transport layer, be it a WSN or MANET, or Internet, until unless there is a too specific technique. I suggest the following reading list.
Computer Networking: A Top-Down Approach Featuring the Internet by Kurose and Ross.
WSN layers broadly are PHY, MAC, NWK and APL. I believe congestion per se is not mentioned anywhere but yes the MAC layer does ensure that the packets are transmitted only when do one else is transmitting i.e follows CSMA-CA. NWK layer just routes the packets as per the algorithm.