I'm looking for something modern, comprehensive, and relatively easy to read (bonus for something that also talks about the Black & White Guelphs). Looking around has turned up Oscar Browning, but I'm afraid that may not be easy to read due to the time elapsed since it was first published. Is there anything better, or is Browning the best source?
You want something that's only about the conflict? The one good example that I can think of off the the top of my head is, L'Italia dei guelfi e dei ghibellini by Sergio Raveggi. Given the fact that the actual activities of these parties were very city specific, the best places to look would be histories of individual cities. Najemy's history of Florence is the best in English.
Browning is waaaaaaaaaaaaaaaay to old.