Garbage Collection and the Ruby Heap is a presentation given by Joe Damato and Aman Gupta at the recent LA Ruby Conference. You only get the slides for now (all 70 of them!), but they're very detailed and can almost work as a standalone concise e-book on Ruby's garbage collection system.

Joe and Aman take a look at C memory management vs Ruby and show the difference between the stack and the heap. As a garbage collected language, Ruby takes the easy route by putting everything on the heap and the presentation demonstrates how the MRI (Matz's Ruby 1.8) does this, as well as how objects are tracked within memory (right down to the underlying C structs).

MRI's garbage collection scheme isn't particularly well optimized (though Phusion's Ruby Enterprise Edition has made some tweaks) and the presentation demonstrates the viability of some other garbage collection schemes (as always, the issue is compatibility with native extensions).

The presentation finishes off with a interesting walkthrough of using memprof (a Ruby memory profiler) to debug a memory leak in Rails 3.

[e-book] The Rails 3 Upgrade Handbook by Jeremy McAnally is a 120 page guide on migrating your apps from Rails 2.x to Rails 3.0. There's a review of it on Rails Inside if you want to learn more.