I followed the exaple described in:
http://wiki.lazarus.freepascal.org/Add_Help_to_Your_Application In order to set an absolute path for the help:

Set BaseURL to file://html/. This will search the html files in the sub folder html. You can specify full paths like file:///usr/lib/yourhelp/

But it does not work, if I set an absolute path:

   helpDB.BaseURL:= 'file:///E:/Temp/help/';
   helpDB.KeywordPrefix:= 'HELP/';
   HelpKeyword:= 'HELP/dragon_graphical_interface.htm';
  the html help viewer always shows the relative path (myapp/help)

andrea