SSH: The Secure Shell (The Definitive Guide)
Barrett, Silverman, & Byrnes / O’Reilly

The SSH Protocol

Version 1.5

Documented in the RFC file from the SSH1 distribution, this version of the protocol is mostly obsolete and abandoned.

Version 2.0 (RFCs)

The SSH-2 protocol is described in five main documents. Architecture describes the overall design of SSH-2. Transport provides a single, full-duplex, byte-oriented connection between client and server, with privacy, integrity, server authentication, and man-in-the-middle protection. Authentication identifies the client to the server. Connection provides richer, application-support services over the transport pipe, such as channel multiplexing, flow control, remote program execution, signal propagation, connection forwarding, etc. Finally, the Assigned Numbers document gathers together and lists various constant assignments made in the other documents.

Other RFCs

These establish various extensions to the core SSH protocols, or related mechanisms.
Version 2.0 (Drafts and Proposals)