If there is just one single development book on your bookshelf, it should be this one - which is saying a lot since Amartya Sen is a personal hero of mine.  I've already read this book twice, I'm sure I'll read it a few more times and reference it on countless occasions.  This is a *must* read.

It's a collection of twenty seven essays written by an all star cast of academics, the majority of whom are economists.  It covers a vast array of development issues written by the people doing research in each area, essentially summing up what we know in a non-technical way for policy makers and development practitioners.

There are just too many gems to list them all: from Angus Deaton on measuring poverty to Acemoglu, Johnson, and Robinson on institutions to the Aghions on new growth theory to Banerjee on Globalization to Duflo on behavioral economic implications to Miguel on ethnic diversity to Urdy on child labor to Kremer on incentivizing r&d for neglected diseases... 

You will read this again and again and learn something every time.