Recently I’ve made a list of all books about retrospectives that I’m aware of for my Retr-O-Mat newsletter. Just in case you also want to explore this topic:

• Agile Retrospectives by Esther Derby & Diana Larsen
  This is the classic. Very valuable! I’ve read it at least twice.
• If you’re German, check out:
  Erfolgreiche Retrospektiven by Judith Andresen
  
  I’ve been to one of Judith’s workshops and really like the activities as well as her additions to the “canon” such as a 0th phase and how to match activities to the team situation.
• Project Retrospectives by Norm Kerth
  AFAIK the first book about retrospectives, albeit in a non-agile setting, i.e. retrospectives last longer and are less frequent.
• The Retrospective Handbook by Pat Kua
• Getting Value out of Agile Retrospectives by Ben Linders and Luis Gonçalves
• Retrospektiven in der Praxis by Marc Löffler

My very own Retr-O-Mat Print Edition is not really book, but just in case 😉

[This blog post was scheduled to be published before the author went on vacation and is valid without signature 😉 See you guys mid-December!]

Update 2016: In the meantime I’ve written an book on the retrospectives, filled to the brim with activities and other advice to improve you retrospectives.