Agile Retrospectives, Esther Derby & Diana Larsen
This is a practical book, written to continuously improve your development project. Based on the authors experiences in real world situations.

Liberating Structures, Keith McCandless & Henri Lipmanowicz
The last few years these guys have gained substantial traction with their development of getting the most when working in and with large groups. The book is a real gain for people who love to read from good old paper. The accompanying website is perfect for looking up formats or digging through all of them.

Getting value out of Agile retrospectives, Ben Linders & Luis Goncalves
Ben Linders and Luis Goncalves put together a nice book on running retrospectives. A little about the how and why of actually investing time in doing retrospectives. Besides the theory there are over 13 retro formats covered. You can download this book free from InfoQ.

Fifty ideas to improve your retrospectives, Ben Williams & Tom Roden
For a mere 10 dollars you can read up on a digital version from Epub and learn from the 50 ideas from Ben en Tom.

Become a retrospective rockstar, 21 different authors
This booklet is also available for free! Download a copy from Retrium and learn from the experiences from over 15 authors.