Assuming you are using Robohelp 11 and generating Multiscreen HTML5 output, you need to make sure your books are not linked to topics. Or change to generating Responsive HTML5 output, which doesn't seem to exhibit this problem.
James - Please do read the sticky topics. Amber has guessed at your output and hopefully correctly. However, posting in the right forum and mentioning the version of RoboHelp would have enabled her to be sure. Perhaps also to have given a different answer.