PUBLISHING TRENDS are curious creatures. First cats, then golf, Nazis and the Holocaust, and now Canadian diplomacy. New York gossip is interesting enough, a slap in the face of an underling gives mild titillation, and at a pinch the humorous anecdotes of a former ambassador manage to provoke the odd chuckle. Sidney Freifeld`s Undiplomatic Notes: Tales from the Canadian Foreign Service (Hounslow, 194 pages, $19.95 cloth) is a sort of "Candid Camera"/"Totally Hidden Video" of the foreign service, with the author -a veteran of Ireland, Mexico and Hungary, among others - delivering the dirt and danger of international representation. Fidel Castro slapped the back of Ecuador`s aged president so hard and so often that the wretched South American leader almost had a seizure, we are told; the brave staff of the Canadian embassy in Moscow were kept awake by a pack of wild and screaming cats; Prime Minister Diefenbaker believed in alcoholic abstinence, and his sensibilities were once disturbed by the name of the confidential messenger Gilbert Champagne. Ponderously decorous and slavishly discreet, the book is sometimes charming and always innocuous, a compilation of the type of stories one resorts to at the end of a very long party. Only for monomaniacs of the punctilios of diplomacy.