Diplomacy: Theory and Practice, 6th ed. – Online updating pages
Chapter 11: Conferences
Nicolson, Harold, The Congress of Vienna. A study in Allied Unity: 1812–1822 (Constable: London, 1946), Chs. 8 and 9. Ch. 9 on ‘The problem of procedure’ is particularly interesting. I must have read my copy years ago because there are faint pencil lines here and there but I had quite forgotten about it.
Wellesley, Victor, Diplomacy in Fetters (Hutchinson: London, 1944), Ch. 3 – succinct and spot on. Certainly still worth reading, and freely available here.