I've sat around a table with members of the Authors Guild a couple times to talk about book promotion. Frankly, it's the only thing of value I felt I got out of the Authors Guild, but one event a year seemed to be the max for Massachusetts (lack of authors?!), so I only lasted two years. When you participate in a discussion about book promotion with 20 or 30 trade published authors, what immediately becomes clear is that there's no one size fits all solution, and a hard-driving author is always at the heart of a success. Some authors spend months at a time on the road doing book readings or seminars, others are relentless at marketing themselves as interview candidates for radio stations, print and Internet media, even TV. Successful authors may even maintain databases and mailing lists of fans to announce every new book release or public appearance.
For self-published authors, the challenge is twofold, because you need to promote yourself as an author before you can promote your book. For trade published authors, the fact that a Random House or McGraw-Hill has published their book is in itself a credential that makes them appear worthy candidates for the standard book promotion methods mentioned above. Strangely enough, this is where being truly self-published (ie, having your own imprint as the publishing company) beats being published by one of the huge subsidy presses like iUniverse or AuthorHouse hands-down. To the media contacts you'll be pitching yourself to, a publisher they never heard of is an unknown quantity that makes a neutral impression. After flooding the market with tens of thousands of titles, the major subsidy presses are known all too well, and the impression is usually a negative one.
Whether you're writing fiction or non-fiction, there's really not that much difference between self promotion and book promotion, in the end, it's all about reputation. If you can establish yourself as a trusted source of a good read or a reliable source of information, you're going to have a much easier time promoting your books, even if the subject matter varies widely. I'm a huge advocate of internet book promotion, but the best internet campaigns are content based. Every new book requires an additional investment of time and effort in generating and posting content, acquiring links, and building the equivalent of a search engine reputation in the new area. The amount of work involved leads many authors to button-hole their careers and stick with the low-hanging fruit.
Building a reputation doesn't happen overnight, unless it's a notorious reputation for doing something awful, so it's best to ignore the quick hits and just start laying stone upon stone. Find a group that shares an interest in your area of writing and participate. There are internet discussion groups for everything under the sun, from mystery novel fans to astrophysics, and if you think you're too good to start at the bottom by reading and responding to posts that strike you as childish, you're never going to rise. If you have a website you're trying to promote, make sure it's in your e-mail signature, but in the end, it's your name people will remember. Until you have a reputation, all you have to sell is your actual writing, and that can be really tough if you can't give people a reason they should take a chance on reading it.