• Increase of economic efficiency of retorting Estonian oil shale : United Nations Symposium on the Development and Utilization of Oil Shale Resources, section IV, [Tallinn, August 26 - September 4, 1968]Väljataga, Juhani1968 https://www.ester.ee/record=b1346522*est
• Room and pillar system of mining in shale mines of Estonian deposits : United Nations Symposium on the Development and Utilization of Oil Shale Resources, section I, [Tallinn, August 26 - September 4, 1968]Talve, Leo1968 https://www.ester.ee/record=b1344839*est