For book/ebook authors, publishers, & self-publishers
Word of mouth is still the best. I know I have the most success when talking and visiting with people at book festivals, conventions, book fairs and school kids. It gives a more personal touch and feel then the net and social media.
Forums and discussions have their place and are effective, but in my opinion, cannot replace a smile and handshake.
author of Allon
There are plenty of widgets that can help turn word-of-mount marketing into viral marketing. Find the widget that best suites your book, marketing strategy and message, then give it to supports. Supports can embed it on their websites, blogs and posts for their visitors to get and embed, and so on. Look for widgets that repurpose existing content to save time (i.e., Twitter, Facebook, blog posts, Youtube, etc.)