Great post, I love the thinking out loud thought process portions.

Some suggestions: try removing stopwords (the, and, etc.), it'll bring out more variation on the analysis. Particularly in the circular graphs.

Try unique n-gram analysis, I suspect that will show something interesting. At the very least 2-gram (bigram/digram) analysis might show something cool.

Some other interesting comparative measures, since the author already has all of the tokens, try the Jaccard index between book pairs to look for similarity. (may even want to break the books down into major sections: e.g. LOTR can be viewed as both 3 and 6 books.

Some others to try, sentence length, word length, distribution of tf-idf scores, etc. etc.

fun fun!