i guess you refer to LaTeX or something, "compiling" the docs on the go when rolling out latest builds. could be possible, difficult to maintain though, as it is a new "language" to learn 
Indeed, I thought about LaTeX, but discarded the idea quickly because it would be way to complicated. A good trade-off between usability/looks and ability to update would be HTML.
We'd have each chapter in a separate file and they would be plain text. Both vital features for submitting changes to SVN (binary formats are absolutely inadequate for version-control, means: no word or anything like it).
When we have HTML, people need to modify the files in a plain text editor, no fancy graphical editors as they (most certainly) would mess up the format and render version control useless.
Most other plain-text formats which could be rendered to PDF would be ok, too, though I'd prefer HTML.
Cheers.
