History and Techniques of the Great Masters: Degas by Linda Bolton. It has an overview of Degas' life and then goes into detail on several of his paintings. In the overview the author mentions that Degas was fascinated with Ingres, "by the end of his life he owned some twenty paintings and ninety drawings by Ingres." They met once and Degas said that Ingres told him, "draw lines, young man, a great many lines."
It's not surprising then that Degas would also be so influenced by ukiyo-e. There is an early portrait of Degas' included in the book, it looks very much like an Ingres portrait.
The book also mentions Degas' working practice. Apparently he would draw and paint his subject numerous times until he had it memorized and then would work out his composition and do the final painting.
Drawing from memory has always intrigued me (I think because I am not all that good at seeing it in my head before I get it on paper). So, I think I'm going to try some experiments based on the line emphasis and careful study mentioned above.
Off I go to draw a great many lines!