Fix embedded books false positives

[swquinn]

When loading books, we are logging false positives on the discovery of "embedded" books.

What is happening is that we're finding the first path with a valid book configuration and then continuing to look through sub directories, and when we find a subdirectory we're saying that we found an embedded book even though there's no valid configuration there.

We should either just ignore subdirectories or, as we iterate over them only alert that we found an embedded book when we encounter a valid configuration file in the subdirectory of a book.