Gene Sharp wrote a very influential three volume book on nonviolence (The politics of nonviolence action). He influenced me (and many others) a great deal. His work underpins many social change movements. I think I first came across him during the Franklin River Blockade.
This article (BBC News – Gene Sharp: Author of the nonviolent revolution rulebook) discusses his influence on the recent events in Egypt and other places. The links with community engagement are clear.
If you want to read From dictatorship to democracy, you can download it for free from the Albert Einstein Institution.