The following files are all in PDF format and are in rough date order (so the stuff at the bottom might be REALLY out of date).
Configuring Corosync with kronosnet explains the new features in corosync that knet brings to us.
Programming Locking Applications is a description of how the DLM works and also covers the programming API. It's applicable to RHEL4, RHEL5 and beyond and is probably the most useful document here ☺
New Quorum Features in Corosync 2.0 Describes the votequorum subsystem in corosync2 (RHEL7) and the new features that help to make the quorum system more flexible and less reliant on the old Quorum Disk system.
Cluster Suite Networking is an overview of how cluster suite in RHEL5 and RHEL6 (including the DLM) uses the network.
Cluster Pic RHEL6 is a single-page diagram showing the RHEL6 cluster architecture.
The Yin & Yang of cman & Corosync describes how cman configures corosync and how to override parameters correctly.
Reading RHEL4 cluster packet dumps describes how to analyze a network packet dump from a RHEL4 cluster.