In 1867 Cassel wrote to a friend: "My greatest delight from early infancy was in Books and matters of former times. Consequently the pursuits of my whole life were bent in that channel. I have therefore amassed an amount of matter that is almost incredible. Among which may be found Letters, Geneologies, and other Manuscripts from many of the most ancient Brethren ... I have travelled thousands of miles and ransacked many old Bee-Boxes and Flour Barrels in the garrets and lofts of their dispersed descendants to collect them...."
As a book collector, Cassel was also an avid reader until his eyesight failed. He wrote articles in the Gospel Visitor using the pen name Theophilus and contributed articles and information to a dozen Brethren periodicals. Although he did not write a history of the Brethren, his knowledge and his library were major sources of information for M.G. Brumbaugh's History of the German Baptist Brethren (1899). Brumbaugh dedicated the book to Cassel.
People came from great distances to use the material in the Cassel Library and to question the book collector and librarian who brought together approximately 50,000 items during the second half of the 19th century. When Cassel died in 1908, newspaper obituaries described him as "the foremost Pennsylvania German bibliophile and a widely known authority on the literature of the Germans in America..."
At his funeral, M.G. Brumbaugh stated, "No man ever lived or will live, who will do for the Dunkard Church what Brother Cassel has done. Our history was engraved and preserved on the shelves of his library. He kept safe our records as a denomination."
Source: The Brethren Encyclopedia