Mark A. Thomson 
The Secretaries of State, 1681-1782. 

Published in 1968: While giving a lucid account of the functions and difficulties of the office of Principal Secretary, the author shows clearly how the retention of this position was a characteristic example of the English habit of clinging to old forms in political matters long after these forms have ceased to bear any relationship to reality. Originally a clerk in the King”s private household and writer of his letters, by the end of the seventeenth century the position had become a political office, second only in importance to that of Lord High Treasurer.