Not specific to soup but:
 
In Amelia Simmon's AMERICAN COOKERY (Albany 1798) there is "to dress a Turtle", a rather detailed discription of butchering and cooking a large turtle.   
 
 
There is a repro of the book in print - not expensive.   
 
        Kevin Richard-Morrow 
 



>I am interested in the history of turtle soup in London and New York City. My impression is that although the dish reached its height of popularity in New York in the late 19th century,  it had long been on menus and it would remain so until the early 1960s.
> 
> I am now looking for sources relating to turtles and turtle soup in New York City.