The main textual resource for Bolzano studies is the continuing production of the volumes of the Bernard Bolzano Gesamtausgabe (Collected Works). These now (as of August 2019) number 102 volumes available in four series and are listed on the publisher’s website (https://www.frommann-holzboog.de/editionen/20?lang=en-gb) where there are details of the series and of which volumes are available and which in preparation.
A particularly important volume for Bolzano studies is the Bolzano-Gesamtbibliographie Bd. E, 2, 3 being a comprehensive set of Bibliographies for the years 1804 – 1999.
A further valuable resource is the website of the Internationale Bernard Bolzano-Gesellschaft at Salzburg https://www.uni-salzburg.at/index.php?id=28292 and the pages Bernard Bolzano-Seiten at https://www.uni-salzburg.at/index.php?id=28321 where there are several further useful links. (See the Newsletter issues for bibliographic sources from 2000 onwards.)
There have been several PhD theses on Bolzano’s mathematics. Two of them Blok (2016) and Fuentes Guillen (2017) are available from the BolzanoinPrague2017 menu at the top level of these pages. The thesis Russ (1981) is available from the Open University or here.