I just bought this book. The author, Phillip Ezolt, gives a really great introduction to all the ways your programs could be limited (CPU, I/O, memory, network), then discusses a number of tools for diagnosing and solving these problems for both individual applications and system-wide changes. He actually describes which parts of these tools are useful, doesn’t just copy and paste the man pages as so many books do.
After that, he walks readers through three examples of identifying and solving different problems with GIMP, Nautilus and prelink. This, for me, was one of the most useful parts. The tools Ezolt uses include oprofile, valgrind, vmstat, gdb, iptraf, sar and many more. If you want to improve the performance of any program, I highly recommend this book.
Before I bought it, I already had read it once while sitting in Borders. It was so good, I bought it despite having already read it.