1) Chris Loosley & Frank Douglas: “High-Performance Client/Server”

2) Cal Henderson: “Building Scalable Websites”

3) Theo Schlossnagle: “Scalable Internet Architectures”

4) Baron Schwartz, Peter Zaitsev, Vadim Tkachenko: “High Performance MySQL”

5) Martin L. Abbott & Michael T. Fisher: “The Art of Scalability: Scalable Web Architecture, Processes, and Organizations for the Modern Enterprise”

Also check highscalability.com for case studies and lots of informative articles about web scalability.