Jonathan Magen has a nice introduction to garbage collection. In it he illustrates various garbage collection strategies by using the same algorithm to get rid of old food in the refrigerator. It’s not a deep technical discussion but is perfect for getting a feel for how the algorithms work and their advantages and disadvantages.
Magen supplies references to more advanced works for those who want to dig a bit deeper. If you’ve wondered how GC works, this is a resource to start with.