A classical book, that still is valid and widely used. In fact, it is the fore-runner to the nowadays very popular Six Sigma craze, but without the Manufacturing focus of that model.

It can be criticized for being more of a consult-manual, than a text-book, so if you are looking for a fix-it-all book, look elsewhere, as you need to be able to have independent thoughts while reading the (admittedly horridly written) text.