I have a few e-books formatted in Microsoft's Compress HTML Help format. To open these files, I had to install a viewer called xchm. To install it in Ubuntu, I opened a terminal shell and typed:
sudo apt-get install xchm
However, the installer did not associate .chm files with the xchm program in GNOME. To fix this, I right-clicked on a .chm file and selected "Properties". I then selected the "Open with" tab and chose xchm from the list of applications.