u/Glad-Owl-8887

i'm looking for a book published most likely in late 2010s - pre 2025 by an author who had at most two other books published before this one. author's initials might have been M L (for some reason i though it was Morgan Llewelyn but it's not- this author is a man). the cover was a pen and ink type of illustration with a very wispy style that reminds me of the old illustrations from Scary Stories to Tell in the Dark. the cover depicted an upper city and a lower slum area with a boy standing below a tall wall in the lower area. the illustration wrapped around to the back cover. the book starts with a list of things that will be in the book (like a content warning/vague spoilers of sorts but stylized for the tone of the book). from what i remember, the plot centers around a boy who grew up with his parents in the slum area and he has a power (i think called a spark or a shine or an itch) that overall will help the very powerful person in the upper city. at the beginning of the book the boy secretly uses his power to pull creatures out of a 'living mud' which permeates through the slums and is what people use to make money down there. his dying father warns him against using his powers so that he won't garner unwanted attention. eventually the boy chooses to be sent to work with the powerful man in the upper city (he has to ride in a cart with other boys from the slums, including part of a gang whose leader goads him into showing his power a little bit). this is what i can remember as i didn't get a chance to finish the book and obviously can't remember the title or author! i hope someone else here can help me out!