one book. I would like to recommend a few more to add to your library.
The first is Green Roof Systems: A Guide to the Planning, Design and Construction of Landscapes over Structure, by Susan Weiler and Katrin Scholz-Barth. If you are looking for a text book approach to green roofs, with fancy charts and diagrams, this is it. Both Weiler and Scholz-Barth are well known in the field. Their experience has made this book standard reading for professionals designing and building green roofs.
If you love green roofs, but live in the city and have no yard, the book for you is: Planting Green Roofs and Living Walls by Nigel Dunnett and Noël Kingsbury. While you have to have something of a green thumb, this book walks you through the steps to green up a wall or roof. There is advice on which plants to choose as well as designs. This book is also a help for landscape designers who have never actually done a green roof.
If you are most interested in the design of green roofs, Steven Peck wrote Award Winning Green Roof Designs. Although it is more directed to professionals, it has great photos and descriptions of some incredible green roofs. It is also a good companion to the Weiler Scholz-Barth book above.
One final book for your consideration is by Caroline Tilston, Rooftop and Terrace Gardens: A step-by-step guide to creating a modern and stylish space. This book is the DIY guide for terraces and the like. It has design suggestions, layouts, as well as detailed instructions. If you live in the city and have roof access, this is the book to liven up your space.
Is this an exhaustive list of books? Certainly not, but it is a representative sample of materials available. In addition to these books, I would like to give a particular shout out to a green roof expert on Twitter, Dusty Gedge (@greenroofsuk). He has great tweets and is worth following.