I think digital agencies suffer most from not having clearly defined deliverables that prospective clients can say yes to easily.
"Built To Sell" has really cleared that up for my own consulting and a handful of others I've helped get their offering better defined.
http://www.amazon.com/Built-Sell-Creating-Business-Without/dp/1591845823
FYI - I've read gobs of books around business models and positioning (Blue Ocean Strategy, Business Model Generation, Influence, Positioning, Blueprint to a Billion, Lean Startup, Personal MBA, etc) and think "Built To Sell" is one of the most practical and useful on the topic.
