I think the manual has been neglected for some time now.
You can probably check the built-in OmniFocus help menu or visit some great third-party sources for OmniFocus:
http://www.usingomnifocus.com
The author has an audiobook or ebook for sale that is worth every penny. If you really want to learn how to use OmniFocus and learn about different workflows that you can use to increase your productivity, this book is an excellent choice.
Click on the Resources link at the top of the page to see a variety of online articles on how to harness OmniFocus' capabilities.
http://www.asianefficiency.com/omnifocus/
A great list of articles written about OmniFocus is located at Asian Efficiency. Each article is nicely written and builds upon each other.
An official software manual will mostly show you the feature set of a program. A third-party writer will show you different workflows and how each feature fits in your own personal workflow. That's the big difference.