Natchaya Chalaysap. (1990) Business memos, letters and telex. National Institute of Development Administration,:ม.ป.ท.Natchaya Chalaysap. 1990. Business memos, letters and telex. ม.ป.ท.:National Institute of Development Administration,;Natchaya Chalaysap. Business memos, letters and telex. ม.ป.ท.:National Institute of Development Administration, 1990. Print.