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Catholic Encyclopedia
http://www.newadvent.org/cathen/03547a.htm
St. Ceslaus, Polish Dominican, d. about 1242.
Catholic Online
http://www.catholic.org/saints/saint.php?saint_id=782
More a travelogue than a biography.
For All the Saints
http://www.saintpatrickdc.org/ss/0717.htm#cesl
Short hagiography of St. Ceslaus.