I have rather specific research requirements for the counterfactual novel I am writing set within an American Revolution which takes a very different course after Washington's defeat and demise at Trenton. Alternate history needs to be true to the details of the times to be convincing where it deviates from the actual course of events. It also needs to be intelligible to a non-expert modern reader, without sacrificing the feel and sensibilites of the period it depicts. This is why, I suppose, no one in costume dramas set in medieval England speak with accurate but unintelligble dialects.
Still, I have had my fill of modern language in the mouths and minds of 18th century characters, which is why I was delighted by the gift of Richard M. Lederer, Jr.'s out of print Colonial American English (1965), a trove of words and phrases found in print in America during the first two centuries of English settlement.
By no means a trained linguist, I still realize that distinctive regions of the American Colonies had strikingly different words and dialects used by people of different classes and origins, and that words in common usage in the 1600s might be obsolete by the time of the Revolution. This book, however, offers many possibilities to judiciously sprinkle into the speech and observations of my characters, and add something of the flavor of their lives and experiences to my descriptions of what they see and do.
One of my characters might deplore the "soul drivers" who herded slaves from market to market, or comment that the introduced European Bee was unknown to the the indians who called it the "Englishman's fly". A "chop-fallen" man would be dejected, while a "flying machine" was not an airship but rather a fast stagecoach.
Buckets were unknown in New England where pails prevailed. Biblical phrases were common points of reference, while pipes, stones and oyster baskets had more earthy meanings. New York's infamous "Holy Ground", a burned over district where most of the land was owned by the Anglican Church, was the largest open air brothel in North America, while strongly anti-catholic New Englanders knew that the "Scarlet Whore" was the papal church.
My next purchase is likely to be Albion's Seed, Four British Folkways America by the estimable David Hackett Fischer. I am also reading up on the Jersey Dutch of the Neutral Ground, as well as the brilliant and troubling Rape and Sexual Power in Early America by Sharon Block. Just as war will warp the values and attitudes of peacetime, what happens to my characters in what becomes a prolonged insurgency of neighbor against neighbor as well as the power of Britain and its German allies will be determined, in part, by their origins, station and gender. Tarleton's Quarter, which became synonymous with giving no quarter at all to prisoners, was not the invention of the men of these times, and the status of prisoners in this rebellion was one of the great policy dilemmas of the Revolution and lead to the deaths by callousness and neglect of tens of thousands of common soldiers and seamen. Forgotten Patriots (2008) by Edwin Burrows is an excellent new book on this harrowing subject and how the memory of these casualties changed in the 19th century.