Bulgaria under Ivan Asen II. a map in Dimitar Rozoff: <i>The Bulgariens in their historical, ethnographical and political frontiers</i>, Berlin, 1917.
Dimitar Rizoff (1862-1918)
<a rel="nofollow" class="external autonumber" href="http://www.bnr.bg/RadioBulgaria/Emission_German/Theme_Geschichte/Material/gk080306.htm">[1]</a>, <i>The Bulgariens in their historical, ethnographical and political frontiers</i>, 1917