William Pitt, Earl of Chatham: The Great Commoner
William Pitt, Earl of Chatham: The Great Commoner (accessible at https://archive.org/details/williampittearlo0000brow) was written by Peter Douglas Brown in 1978.
I have not actually read this, I'm only using it to substantiate how Frederick was a patron of the opposition. He hosted an alternative court at Leicester House. He provided positions and incomes to Pitt and George Lyttelton, and generally associated with Patriot Whigs such as Grenville.