Have some green tea!

The Little Book of Semaphores by Allen B. Downey

is a free (in both senses of the word - as in "free speech" and also as in "free beer") textbook that introduces the principles of synchronization for concurrent programming. The site also contains a 1 hour introduction to semaphores presented at Northeastern University by the author. Theory, examples, exercises, puzzles, that's the way a good CS course should be.
"The approach of this book is to identify patterns that are useful for a variety of synchronization problems and then show how they can be assembled into solutions. After each problem, the book offers a hint before showing a solution, giving students a better chance of discovering solutions on their own."