The books by Jackson and Clayton are both excellent, and they complement each other.
The Drogin book is superb from a practical calligrapher's point of view, not just for the exemplars (which are highly informative, but frankly of middling quality), but also for many reproductions of manuscripts which are hard to find elsewhere - certainly in a single volume.
Drogin's paleographic content though is a little eccentric, and if that's important I'd recommend
A Guide to Western Historical Scripts from Antiquity to 1600 by Professor Michelle P Brown which is definitive, reasonably accessible, and equipped with an exemplary bibliography.
What you could really do with to round out the reading matter would be a book which covers the development of Humanistic and Italic scripts and their transition to roundhand in some detail. That's not my area, and nothing leaps to mind. Alfred Fairbank's "A Book of Scripts" is a slim volume and out of print as far as I know, but good on the history nonetheless. As for 20th century and contemporary calligraphy, I hope you'll report back if you find an exceptional book, because thus far I haven't. Plenty of interesting ones, but nothing stands out.
