Over the weekend, en.Wikibooks got a new namespace, the "Subject:" namespace. it's goal is to be an organizational namespace, where we can list and cross-list books based on a variety of metrics. By using a comprehensive set of categories, templates, along with clever use of special pages and DPL, we can automatically generate a large number of helpful book lists. The only problem so far is that there are so many possibilities, that nobody quite knows where to start.
Also, it will take much more effort to create templates and categories, and then properly tag all the books, to help maintain all the millions of lists that we are likely to develop.