Ruzicka, Leon. Die Münzen von Pautalia. (Sofia, 1933).
The standard reference for the coinage of Pautalia, listing 909 varieties from Antoninus Pius through Caracalla and Geta. 216 pages, 10 plates. Long out-of-print and very difficult to find. Reprint available from Barry Murphy.
Also see:
Ruzicka Serdica