We're not entirely clear on why the links preprocessor has to be done
*before* rather than after, but we assume it is probably that as a
builtin preprocessor it does some processing on the raw book source,
and not just the JSON data.
Also a real use for Python pattern matching? I know I was surprised too.
Change-Id: Ibe8b59e7b5bd5f357a655d8b4c5f0b0f58a67d6b
