Book content is now served under /content/book/...
The old access to book content via a top-level URL /book/... is so far
preserved for backward compatibility.
Redirects were changed to use the new URL scheme. Links in the search results
still use the old scheme.
One important missing test is that the content of the customized
resource is read from storage every time rather than once. Testing
that requirement would involve creating temporary files which is a
little more work.
