Hi Herbert
Thanks for reporting the issue.
For this problem, I will suggest two solutions
1. Either create a master book containing all your books / fm documents and then publish, you will get integrated Responsive HTML5 output.
2. Or use TCS and in RoboHelp use merge project Adobe RoboHelp 11 * Merging Help projects
Please let us know if you find any issue in that.
Thanks
Amit Jha