Well fantastic fiction (which is my go to resource when I'm working at the library's fiction desk) does not suggest that is part of a series. Perhaps the annotation is erroneous?
http://www.fantasticfiction.co.uk/s/henryk-sienkiewicz/
[edit]: Looks like this book was also published as The Teutonic Knights sometimes in two volumes. Is it possible what you read was just the first volume?
[edit2]: This thread probably belongs in some other forum but I don't frequent any of the others so I have no idea which one...