William Henry "Billy" Reed (29 July 1875 – 2 July 1942) was an English violinist, teacher, minor composer, conductor and biographer of Sir Edward Elgar. He was leader of the London Symphony Orchestra for 23 years (1912–1935), but is best known for his long personal friendship with Elgar (1910–1934) and his book Elgar As I Knew Him (1936), in which he goes into great detail about the genesis of the Violin Concerto in B minor. The book also provides a large number of Elgar's sketches for his unfinished Third Symphony, which proved invaluable sixty years later when Anthony Payne elaborated and essentially completed the work, although Reed wrote that in his view the symphony could not be completed.
His name appears in various forms: William Henry Reed, W. H. Reed, W. H. "Billy" Reed, Billy Reed and Willie Reed. He was known to his friends as Billy.
William Henry Reed was born in Frome, Somerset. He studied at the Royal Academy of Music in London under Émile Sauret,Frederick Corder and others, graduating with honours.
He first met Edward Elgar in 1902, as a violinist in the Queen's Hall Orchestra. On 17 January, Elgar has just completed a rehearsal of his incidental music to Grania and Diarmid with the orchestra, when Reed approached him, introduced himself, and asked whether he gave lessons in harmony and counterpoint. Elgar said "My dear boy, I don't know anything about those things". They did not become personal friends at that time; however, their paths continued to cross in the course of their work. Reed was a founding member of the London Symphony Orchestra in 1904. His physical appearance was quite similar to that of Elgar's close friend August Jaeger (the "Nimrod" of the Enigma Variations of 1899), and that may have played some part in Elgar's always having something positive and encouraging to say to Reed whenever they happened to meet.