Haydn ventured into a bookshop and asked for a good textbook on theory. The bookseller named the writings of Carl Philipp Emanuel Bach as the best and most recent. Haydn wanted to look and see for himself. He began to read, he understood, found what he was looking for, paid for the book, and took it away thoroughly pleased. That Haydn sought to make Bach’ s principles his own, that he studied them untiringly, is apparent even in his youthful works from that period. From his nineteenth year Haydn wrote quartets which gave him a reputation among lovers of music as a profound genius, so quickly had he learnt. As time went on, he acquired Bach s later writings. In his opinion Bach’s writings form the best, most thorough and most useful textbook ever published.
As soon as Haydn s musical output became available in print, Bach noted with pleasure that he could count Haydn among his pupils. He later paid Haydn a flattering compliment; that Haydn alone had understood [Bach’s] writings completely and had known how to make use of them.
from: AC Dies, Biographische Nachrichten von Joseph Haydn, Vienna, 1810, R/Berlin, 2nd edition, 1962, pp. 40f.
The dark, dramatic, improvisation-like passages that appear in some of Mozart’s and Haydn’s works are due in part to the influence of Carl Philipp Emanuel Bach’s work.