Hmm, it's difficult to say. As mr_nobody found, it seems to have been published as both one and two volume editions. It seems to be in one complete volume at Project Gutenberg, here's the link to all works that they have by him. It appears to match the list of his works on Wikipedia
http://www.gutenberg.org/ebooks/sear...ads&query=1052
http://en.wikipedia.org/wiki/The_Knights_of_the_Cross
http://en.wikipedia.org/wiki/Henryk_Sienkiewicz