This book has just been published by Evangelical Press. I am probably biased because I have written a chapter in the book on Keller's doctrine of the Trinity, but this book is long overdue. Having read the whole manuscript, it is a very stimulating read. It is not just about critiquing Tim Keller's public teaching, but it provides a stimulus for each of us to evaluate our own theology and doctrine.
The editors are Bill Schweitzer and Iain D. Campbell. It can viewed at this link: http://www.amazon.co.uk/Engaging-Keller-I-D-Campbell-Schweitzer/dp/0852349289/ref=sr_1_1?ie=UTF8&qid=1372103889&sr=8-1&keywords=engaging+with+keller