Imagine if Forrest Gump developed a famous Web site. Imagine if one day he sat down on a park bench to tell you how he did it. Now, imagine it with a strong English accent ;)
"Building Scalable Web Sites" is a virtual brain dump of everything technical that you need to know in order to build something like Flickr. (The author, Cal Henderson is the engineering manager for Flickr.) If you're a new engineer, it might be a godsend. If you're a more senior engineer with little time and less patience, it can be incredibly frustrating.
I found myself alternating between these two extremes. I was tearing my hair out when Cal spent five pages explaining what source control is and listing its basic features. I questioned relevance when Cal explained how to write an HTTP client from scratch in PHP (p. 143), and I was baffled when the very next section randomly jumped to the topic of "Remote Services Redundancy" (p. 145). On the other hand, I was grateful when Cal showed me how to use Ganglia as a wrapper for RRDTool to collect and graph application-specific metrics.
This book lacked O'Reilly's standard polish. For instance, in one code sample, the indentation didn't match the braces (p. 148). There's a simple typo, "abilityto" (p. 108), that shouldn't have made it past a spell checker, let alone an editor. It makes me wonder if O'Reilly was rushing this book out the door because Flickr is hot right now, or perhaps "translating" Cal's British English was burdensome on the editors.
In summary, about 30% of the material was new to me. I patiently marched through what I already knew, and I managed to learn a few new things, for which I'm thankful.