Nate Berkopec is the author of the Complete Guide to Rails Performance, the creator of the Rails Performance Workshop, and the maintainer of Puma, the Ruby web application server.
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.
I've been working with Rails for years but haven't had a lot of experience configuring servers or tuning performance. It's usually either been handled by someone else or I've just copied and pasted a template for a small hobby application. Book did a great job filling in holes in my knowledge and offer clear practical advice.