I am interested in the stress, tone and pitch (STP) aspects of historical linguistics. How do phonetic and other types of changes affect STP changes? How do languages end up with entirely different accentual systems? An example of that (according to Wikipedia) is from PIE to late Proto-Germanic with a transition from pitch accent to predictable stress accent.
You might try the Oxford Bibliographies entry on tone, by Remijsen, if you can get access. There is a section on tonogenesis, though that is the opposite of the historical trend that you are asking about. As far as diachronic change is concerned, there is not much written about tonoexodus, but this article by Ratliff could be helpful. Change from tone to non-tone languages occurs in Bantu and Atlantic languages. If you are specifically interested in so-called "pitch accept", you might want to look at the literature on Japanese and Basque, or dialectology of continental Scandinavian, Baltic or South Slavic.