If you ever "go live" and start getting the "oh sh#$" queasy feeling, you need to read this book.
To be perfectly clear, this is not a software design patterns book in the mold of GOF, POEAA, POSA etc, though it is chock full of failure scenarios and how to handle it. This book is not about how to pass QA(zero high priority bugs & deliver all functionality), but how to design your system for the real world. When your user base multiplies by a factor of 10 and you still need to handle the 5 9s; when an external dependency fails or responds sloooooowly; when you run out of threads etc.

Buy it, read it and become a better engineer.

-Venu