I'm also very interested in the history of the cocker and have many old books,my latest purchase is The Sporting Spaniel by C.A. Phillips and R. Claude Cane, first edition was published in 1906 my copy is 1924 and gives the history of all the spaniels from the 19 hundreds, it makes interesting reading on the evolution of springers and cockers from field spaniels, who look nothing like the field spaniels of today, on looking at the pictures of the very old dogs it is hard to tell the difference between them and there is a picture of a springer that looks very like a modern working cocker, there is also a picture of a black field spaniel bitch called Rona born 1889 and she looks like today's working cocker. The one thing that Phillips mentions is that cocker's have low set ears and I've wondered were the higher set of ear in the workers originates from as the set of the ear is more like a springer. In The new Encyclopedia of the Dog by Dr. Bruce Fogle there is a picture of a modern field spaniel which states that by the end of WW II is was almost extinct, but by 1969 the numbers had safely increased as in the 1960s, English cocker and Springer Spaniels were used to regenerate the breed, producing today's affectionate dog.