The Complete Guide to Rails Performance
Delivers exactly what it promises in a well-defined scope: A comprehensive, end-to-end review of how to tune Ruby and Rails applications. This'll be my first recommendation to interested in or working in that field. I especially like that it's purely focused on performance, and doesn't conflate it with scalability whatsoever.
The extra star would've been if the section on measuring the impact would've been more substantial. There's lots of references to companies boosting conversion, but it's unclear how it's measured. Revenue and conversion can be difficult targets to measure against unless the experiments are extremely careful. Additionally, the section on writing acceptance/user tests felt a bit weak. The approach taken seems to be something that'd age fairly poorly, which I'd say the author acknowledges, but I was hoping for more here.