Working on chapter 11 which has one download the shovell-debug-01 project and run it to work on debugging. When I try to run it ruby script/server I get an error about missing the Rails 2.0.2 gem. I appear to be running 2.3.4. I've tried commenting out the line in environment.rb and changing the version but I still get all kinds of errors when trying to start the server. I don't really want to mess with downgrading unless it can be easily reversed once I'm done with the exercise.
I have the same problem. Good to know I'm not the only one. I tried the same thing you did and got the same errors. Unless someone wise can point out a way out of this, my backup plan is to cheat a little and find out what the bug was by reading the bug and introduce the bug back into my own Shovell code and try out the debug functionality.
If it were me and I wanted to use the downloaded source files from the book, I'd make sure the Rails environment matched that used for the book. It might even be worth using a virtual machine (via VirtualBox for example) for this as it makes it easy to set up a clean install without affecting anything else on your PC.
You can fix the gem errors, but a lot has happened between version 2.0 and 2.3, and I think this won't be the last error you will run into. And while you are learning, the last thing you want is to be unsure whether something isn't working because you've misunderstood/misentered something or it isn't working because of a version clash.
And I wouldn't worry about learning with a slightly older version. The basic concepts and the majority of the workings of Rails are the same as those presented in the book. Once you've mastered the book, it is only a small step to get up to speed with the recent changes.