The Communication Mechanism (Volume 1)
Chapter 15: Unix Domain Protocols Learn how to send data and file descriptors between processes on the same machine.
Thread Creation Basics (Volume 1)
Chapter 26: Threads Learn pthread_create and how to write a basic Threaded TCP Server.
Basic Synchronization (Volume 2)
Chapter 7: Mutexes and Condition Variables The mandatory basics. You cannot write safe threaded code without Mutexes (locking) and Condition Variables (waiting).
Advanced Synchronization (Volume 2)
Chapter 8: Read-Write Locks Use this when you have a server where many threads read data (like a lookup table) but few write to it. It is a critical performance optimization over Mutexes.
Chapter 10: Posix Semaphores Use this to count resources (e.g., limiting your server to processing only 5 heavy tasks at once) or for signaling between different processes.