Research and Advanced Technology for Digital Libraries 21st International Conference on Theory and Practice of Digital Libraries, TPDL 2017, Thessaloniki, Greece, September 18-21, 2017, Proceedings - Kamps, Jaap; Tsakonas, Giannis; Manolopoulos, Yannis; Iliadis, Lazaros; Karydis, Ioannis (Springer International Publishing : Imprint: Springer, 2017)