First, the SVN url has changed, so the example in the book won’t work - but second: when you use the SVN path provided at http://www.liquidmarkup.org/ it will end up installing under /vendor/plugin/trunk - not /vendor/plugin/liquid :(
So I just went ahead and installed the gem instead.
sudo gem install liquid

Sounds like a good idea. I’ll see what I can rustle up. Also should show the previous posts on the reply page…