I recommend you read this book:
You'll be glad you did.
The suggestion by James is great. Here is another good option that you can look into:
Great foundation for the basics of marketing.
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.
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.
The best marketing book I've ever read is Buzz Marketing. It's not quite about marketing plans, but it will expand your understanding of marketing and help you to think outside of the box to find effective ways to generate awareness and interest for your business.