I havn' t tried the box calls yet but here is a pretty informative page....

http://www.geocities.com/theboxcallpage/

Also http://www.customsawing.com sales " Neil Cost" poplar blanks for long boxes. They also carry pre-molded paddles in a good many woods and also have hardware for box calls.

Neil Cost had a book on the subject but you might be hard pressed to find one and if you do it' s going to be $$$$$, I think it was called Talking Turkey