Reddit reviews Chinese Martial Arts: A Historical Outline

If literary sources are available and you're interested enough to spend a few bucks on it, there's quite a bit of stuff available. In case you haven't already done so, consider changing "kung fu" to "Chinese martial arts" in your search queries. For example, I just did search that on Amazon Kindle store, and this book (which, admitted, I haven't read but probably will buy...) is five bucks on Kindle.

For internal arts, see Brennan Translation, but however cite the original sources with a note "translated by Paul Brennan".