After having written nearly a dozen technical books, I know the value of an outline. In fact, in some ways, developing the outline is one of the more difficult aspects of writing a technical book. The outline is so important, that it actually becomes part of the book contract with the publisher. Because it is so much work, I try to make the outline as useful as possible, and not do as I did in Freshman English, write a sort-of outline after I finished writing.
Once I complete my outline, I send it around for peer review. I ask my peer reviewers if the outline looks like it adequately covers the topic, if the emphasis on certain subjects is correct, and if there are any glaring missing items. I often receive some great suggestions during this phase of the project. One of the great things about writing multiple books on a specific subject is that I develop a collection of great reviewers. I also know which reviewer will provide excellent feedback on which aspects of a project. So, if in the past a particular reviewer has not provided meaningful feedback during the outline phase, I do not bother asking them in the future.
One of the things I do, that has proven to be really helpful, is that I keep extending the outline. I add additional levels to the outline, and continue adding levels, until I am at the paragraph level. Once at the paragraph level, it is a simple matter to go ahead and write the introduction sentence to the paragraph. At that point, the book nearly writes itself.