Agile may be the best-kept management secret on the planet and if you want a quick-start introduction, then Agile NOW is essential reading as it is a quick-read, accessible and easy to dip into - yet covers all the essential theory and provides practical advice. 

Agile is a different way of thinking thats steeped in common sense and produces immediate results. It will help you design better products, get faster results, cut down costs, and keep improving as you go. With a simple system called The Golden Triangle - 1. Prioritising, 2. Time Boxing and 3. Change Management - you can hit the ground running and get started immediately.