22 Immutable Laws — Violate Them at Your Own Risk! by Al Ries & Jack Trout

It’s a short quick read and but I think you’ll come away with one or two key lessons that with stick with you. Personally, I found the valuable lessons were a bit front loaded (the first couple laws are the best). In short: turns out the best “marketing” decisions you’ll make come long before the paint is dried (or even applied) to the product.

All of the Al Ries books are excellent; don't be scared by some of the dated anecdotes, the principles still apply.

A must-read classic (along with the separate "Immutable Laws..." series ones on Positioning and Branding). Their warnings against Brand Extension still get violated all the time by big companies for the very reasons they describe (internal Groupthink around "our strong brand...", when most of those people don't understand how branding works in the consumer's mind...), e.g. by the likes of Microsoft, Google, etc.