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This is yet another reason why Omni's apps are worth their price
This manual is really well written and covers what you need to know about OmniGraffle and even more. Highly recommended. Even though OmniGraffle is intuitive enough to be used without a manual, reading this book shows you small tricks and workflows to make you even more productive.
Thanks to the OmniGroup for investing the effort in writing such detailed user documentation.