Have you installed any new programs recently if so maybe the file association has changed for html files.
Right click the html book you want to open and click OPEN WITH, select the option CHOOSE PROGRAM.
When the list appears scroll down till you fin your default browser click it once then make sure the option underneath called ALWAYS OPEN WITH THIS PROGRAM is ticked.
Press ok and it should open in your browser.
If your using internet explorer I would strongly recommend getting Mozzila Firefox and making it the default browser. But try the above first.
Hope that helps