Alas, I have to pass this on to mildred_of_midgard, because being German,the Frederick biographies I'm familiar with are as well. In the meantime, why not check out Mein Name ist Bach, a movie based on the historical encounter(s) between Frederick and, well, Bach? That one has English subtitles, is focused on a brief point in time (so you don't have to know context beyond the one the movie provides), and it does give you a first impression of Frederick's personality. (Though, as Mildred said, the jerk woobie part, not the magnificent bastard part, but that's due to circumstance. (The magnificent bastard part comes out when he's facing off with most of Europe, not with musicians, some of which depend on him for a living.)