Several Merb books were in progress when the Rails/Merb “merbger” was announced. Although most of these are being re-purposed to cover Rails 3, their current content is still Merb-specific:

Beginning Merb

Matthew Ford (Apress)

Merb: What You Need to Know

Aaron Farnham, Ben Burket, and Brian Smith (Apress)

Merb in Action

Yehuda Katz (Manning; Beta PDF)

The Merb Way

Foy Savas (Addison Wesley Professional; Beta PDF)

There is also a community driven, open source book available on Github:

http://github.com/mattetti/merb-book/