Petkov, Krastyu
Routledge , 1991
Buku Teks
edited by Maggie Toy
Academy Editions, 1996
Buku Teks
Westview Press, 1985
Buku Teks
by the Economist Intelligence Unit and the Cartographic Dept. of the Clarendon Press
Oxford University Press, 1956
Buku Referensi
Millard, Thomas F., author
Century, 1916
Buku Teks