Complex sentence: Surviving monks fled north across the Sava River and found refuge in the Orahovica Monastery in Slavonia, bringing with them a number of their manuscript books, which thus became part of the Orahovica library.
Split into simpler sentences: Surviving monks fled north across the Sava River and found refuge in the Orahovica Monastery in Slavonia. They brought with them a number of their manuscript books, which thus became part of the Orahovica library.