I've seen three great Metasploit books published lately. The one that most people are probably already familiar with is Metasploit: The Penetration Tester's Guide by David Kennedy, Jim O'Gorman, Devon Kearns and Mati Aharoni. The book is very comprehensive, and packed full of great advice. David Kennedy is Chief Information Security Officer at Diebold Incorporated and creator of the Social-Engineer Toolkit (SET), Fast-Track, and other open source tools, so he really knows his stuff. By the way, Rapid7's HD Moore wrote the foreword for this book. (Get sample chapter and 30% discount code)
The other two books were both published in German, but I know that Metasploit has a huge following in Germany, so I want to include them as well. The first one is Penetration Testing mit Metasploit: Eine praktische Einführung by Frank Neugebauer. It's a perfect book if you're just getting started with Metasploit and need a good primer. Frank is working for the Computer Emergency Response Team der Bundeswehr (CERTBw) as an IT Security Specialist. (Get sample chapters)
There's also Metasploit: Das Handbuch zum Penetration-Testing-Framework by Michael Messner, a much more comprehensive book that is a great reference work for advanced Metasploit users. Mike works as a Senior IT Security Consultant for Integralis in Germany. He conducts security assessments, penetration tests, and gives trainings for Metasploit. I'd like to thank Mike for giving me the opportunity to write the foreword for this great book. (Get sample chapters)
In case you'd like to order your copy of these books, I've linked the covers to the Amazon order page.