@baf, thanks for the reply. i changed the kindlegen permissions to 777 but don't think i want to start messing with chown. i'm on PHP 5.4 so can't safe_mode_exec_dir().
errors weren't being reported on the remote server, but moved to local and now get
Code:
Error(prcgen):E21008: can not write to output file. If the file is being used by any other application, please close it.
i'm guessing that kindlegen doesn't have sufficient permissions to write the mobi file, or that it's trying to write the mobi first to "/tmp/mobi-3VEzNm" rather than to the cwd
getting closer