Thanks to Marla Raucher Osborn, who shared a photo of a book about Kańczuga with me on Facebook. That book, Kańczuga. Miasto i gmina. Dziedzictwo kulturowe, is about the cultural history of the town, and includes a section on the Jewish cemetery in Siedleczka. When trying to track down this book, I found that the publisher, Podkarpacki Instytut Książki i Marketingu, has actually published four books about Kańczuga, although all are out of print. Those books are:

Kańczuga. Miasto i gmina. Dziedzictwo kulturowe (Kańczuga. Town and Municipality. Cultural Heritage.)
Kańczuga. Miasto i Gmina (Kańczuga. Town and Municipality.)
Kańczuga i okolice. Przewodnik (Kańczuga and its surroundings. A Guide.)
Historia Miasteczka Kańczugi pisana 1889 r. (History of the town of Kańczuga. Written 1889.)

The first three seem to essentially be guidebooks. The last one is perhaps more interesting, as it is a re-print of a book written in 1889 about the history of Kańczuga. It was written by a priest, and apparently was based on many documents he found in the church there, so I have no idea how much, if any, deals with the Jewish community, but being as it was written when the Jewish community was still a major portion of the town, it would be interesting to read.

Of course, all of these books are written in Polish, and hard to find on top of that since they’re all out of print. If I knew we had someone willing to translate the important parts of these books, I would try getting copies of them. If you can read and translate Polish, and want to help out, let me know.