Do not use version 7.1
I just downloaded the KChmviewer 7.1 portable and noticed that the program ignores several books in the .chm files I opened.
I've been able to open the ignored .chm files in other .chm viewers, including the built-in .chm viewer which Windows 7 and XP both contain and the books are displayed just fine, which leads me to assume the fault lies in the KChmViewer.
My advice is not to download the latest version and get the 6.1 version, which can display the .chm books. I was fortunate to have the 6.1 version, but maybe it can be made available on this site as well.
Apologies if there is a feature which allows users to retrieve older versions of portable apps.
Hopefully the bug will get fixed in time.
At least, this app can be downloaded from the PortableApps.com/SourceForge.net repository:
http://sourceforge.net/projects/portableapps/files/KchmViewerPortable/