The Little Book of Semaphores: A Reader's Guide
Allen Downey's free OS concurrency text — semaphores, threads, and puzzles.
Allen B. Downey's The Little Book of Semaphores teaches concurrency with semaphores through stories and code puzzles — classic CS teaching text.
Topics
Mutual exclusion, producer-consumer, dining philosophers, deadlock intuition.
How to study
Work exercises; draw state diagrams — theory sticks through problems.
Best alongside an OS course or parallel programming intro.