Been reading Pro ActiveRecord by Kevin Marshall, et. al., in preparation for moving our PHP webapp to Ruby. It's a small book, and arguably one could just read the ActiveRecord source, but let's face it: no-one's going to print that stuff out and read it on the subway.
For anyone doing anything serious with ActiveRecord, such as welding it to an existing, properly-defined (as opposed to rails-auto-created) database, this book is highly recommended. Clear, concise, and thorough coverage with good examples.
I haven't read the chapter on legacy databases yet, but I know from the first 5 chapters that I can make ActiveRecord do what I need it to without too much trouble, which helps a lot in planning this project.