I recommend checking out Enterprise Rails by one of my former coworkers, Dan Chak: http://www.amazon.com/Enterprise-Rails-Dan-Chak/dp/0596515200
He describes a lot of the work we did at CourseAdvisor to scale our site to handle millions of users. I think Rails has evolved to a point where some of the approaches he describes are not the best, but I think it still contains some very helpful advice and strategies. (I'm not involved with the book in any way so this is not self-promotion).
Courtney