Jacob was a prolific composer. "Grove" lists 16 concertos by him for a wide variety of solo instruments, including trombone and timpani. A website dedicated to Jacob lists more than 700 original compositions or arrangements of existing music. His biographer (and former pupil) Eric Wetherell writes that as a composer, Jacob was influenced more by early 20th-century French and Russian examples rather than the German tradition. Wetherell writes of Jacob's "clarity of structure and instrumental writing that shows a keen awareness of the capabilities and limitations of every instrument". Reviewing a concert of his music given in 1939, "The Times" said, "As a general description, 'Good, but a little dry' might be justly applied to Jacob's work".