No memoirs by Somerset but some letters - including some relating to Waterloo in The Waterloo Papers edited by Edward Owen (AQ & DJ Publications, 1997). His letters home to his brother, the Duke of Beaufort and his sister-in-law the Duchess are still at Badminton. I've seen some of them and they are not quite as interesting as you might hope.

And, of course, there is John Sweetman's biography, which again is useful, but not quite as rich as one might wish.