Within minutes of reading Paul Dean's excellent article of March 31 on the Dordogne, and Gloria Lopez's sensitive piece on Mme. Gorse and Le Coux et Bigaroque, I shot off a letter reserving a place for my wife and me for this July. Now that I am actually here, I want you to know that Dean and Lopez were right on target. Castang is a kind of oasis in Paradise. The inn where one can speak English is a restful change for the weary American trying to remember high school French, and paradise is the marvelous food, gracious ambiance and, of course, the Dordogne Valley itself.