I'm reading "How to Think Like a Computer Scientist: Learning with Python" from this site:
http://openbookproject.net/thinkcs/python/english2e/
When viewing a page, I get the message:
"Additional plugins are required to display all the media on this page."
When I click "Install Missing Plugins ..."
It says "Available Plugin Downloads" and under that it says "Press next to install these plugins."
When I click "Next", it says "Completing the Plugin Finder Service", and under that it says "No suitable plugins were found."
It doesn't seem like there's anything missing from the pages, but I don't know.

I guess you mean the plugin to view svg graphics.
Slackware has the need library libmozsvgdec.so in the lisvg package, so you have to look for something similar for your distro.

Regards

I know that my browser is capable of displaying svg as it is, and I have the latest version in the repositories of librsvg installed.
It's very strange. It doesn't say exactly what I don't have, since all it says is "no suitable plugins found". I haven't received a message about needing a plugin and had it come up with no suitable plugins since I started using Ubuntu.

What browser is it? I suppose it's Firefox. And I suppose you need the svg plugin, cause the only thing I saw visiting some pags in the site you posted, are some svg graphics. You could post the exact page that gives the missing plugin message if it's not svg.

Write "aboutlugins" in the location bar and see if you have the svg plugin (the libmozsvgdec.so library). You can copy libmozsvgdec.so (or create a smylink) in your browser's plugins directory.