Recently I started adding small amounts of ground coffee in my breads. IMO, the coffee flavor marries well with sourdough. It behaves much like dark malts (chocolate and other such malt) in that a small amount colors the crust and crumb a nice earthen dark brown. I’d start off with only 2.5%. Coffee is very potent in bread. If you are able to grind fresh, great. If not your favorite pre-ground coffee blend will work fine.
The enhacnced color and flavor is noticeable and distinguished.
Even though the temperature got away from me and the dough over-proofed, this bread was a treat to eat!
Dan