The answer depends a lot on what you have around you and what your needs are; assuming that
- You don't have extensive needs beyond Latin-1 and Math character sets, or simple use of Unicode character sets
- You don't have a need for overly-rich or complex page layouts (i.e. you're not doing page layouts that you'd see in a glossy magazine)
- You don't have external format/structure requirements that would conflict
- Your main operating environment is Windows
Then, Adobe FrameMaker is probably the best, first choice for long, technical documents.
Adobe has over the past few years orphaned pared off all the previous supported platforms for Frame (the 68K/PPC MacOS, brief flirtation with Linux, various Unices one at a time) to the point where it's not worth considering if you're not on Windows. (And sadly, historically speaking, I thought the branch of Frame they developed on Windows was not nearly as robust or as easy to use as the version of it on Mac, or Unix, but things may have improved now that they're really only supporting one platform for it.)
I have never used any tool that makes the writer's job as easy, end-to-end. There are better tools for page-layout, better tools for just the writing end of things, better tools for large scale, structured content-management, but if what you're trying to do is write a sizeable technical document, from scratch, and be able to produce reasonably flight-ready PDF you can pass to a publisher or print-house, then FrameMaker has been and still is pretty peerless.
DocBook is a specification for document structure more than it's a software stack, so writing with the DocBook structure would still require you to have a toolchain of some sort. The version of FrameMaker that supported structured editing did, I believe, support using the DocBook structure and let you produce SGML output instead of, or in addition to, "printable" output (i.e. PDF or PS). However, using Frame's structured features were, in my experience, significantly challenging and finicky: unless you have a firm requirement for fully structured source, or for passing DocBook to your publisher system, I'd question the need for it in your shoes.