A while ago, I settled on DocBook/XSL as my authoring system. I use an extension of DocBook that I wrote which makes my personal style of structuring a CL project reference guide easier to build. It also provides some conveniences for the particular way that the syntax of CL functions/macros/etc are shown. The extensions are a set of XSL templates implementing some higher-level constructs that go along with the DocBook stylesheets; these get transformed into the DocBook vocabulary for later processing.
Here's an example of the help source for a function called MAKE-POINT in a package called LPC, using the extension vocabulary that I mentioned above:
<function name="make-point">It's possible to mix tags from my extension vocabulary (e.g., function, syntax,
<itemizedlist mark="bullet" spacing="compact">
A <refclhs>list</refclhs> of alternating x/y coordinate values.
An <refclhs>integer</refclhs> x coordinate value.
An <refclhs>integer</refclhs> y coordinate value.
This function returns a <refclhs>vector</refclhs> representing one or
reftopic) with DocBook/XSL tags (e.g., itemizedlist, listitem).
This is what the end result looks like:
The output format is HTML Help, so the above screenshot shows part of the help viewer (it doesn't show the search/index tab nor the toolbar). My development platform is Windows XP. Not that there is anything wrong with portable solutions, and you know, raw HTML is often as good a choice as anything else. But HTML Help has features that I want, it's fairly customizable, and I can't think of many situations where a Windows user wouldn't have the viewer. Underlying this is the fact that I generally assume a Windows-based development environment for my work.
One future improvement I'd like to explore some day is to extract CL doc strings from the code and merge that in with other help content. I hear the SBCL developers do something along these lines to generate their manual, one difference being that I believe they are using texinfo or at least migrating in that direction, so I ought to take a look at what they do.