The man page documents the useful parts of the disassembler.

The included interp.rb Ruby script can be used to extract action and preaction
routine addresses from the output of infodump -g, allowing zd to disassemble
these routines (it cannot find them on its own).  The resulting file can be
passed to zd's -A flag.  For example:

infodump -g zork1.z3 | ./interp.rb | ./zd -a 0 -A - zork1.z3