Documentation is currently undergoing revision, albeit in fits and starts.
Quick Start guides for Relying Party and CA on Ubuntu Xenial are
here
The User's Manual. This is old, and corresponds, roughly,
to the old-trunk
branch. Portions of it are now obsolete, but it
may still be useful as a general guide to some of the more obscure
bits of the software.
If you find a place where the code and the manual disagree, the code
is probably right.
Content salvaged from old trac.rpki.net wiki. This is
mostly for archival, although some pieces of it may still be directly
useful.
Not written yet. At one point we experimented with Doxygen, but the
results were not particularly useful. But that was years ago, and
Doxygen has undergone a lot of development since that time, so it
might be worth another try.