In this case, it would seem doubly imperative that you contact the author:
1) There's the standard concern about obtaining permission before you use his/her intellectual property, and:
b) Translation brings additional concerns. For many works, because of the unique structure, vocabulary, and conventions of each language, there's no possibility of a perfect translation. To translate Shakespeare, for e.g., it isn't enough to get the words right; they also have to fit a metric pattern, and often they should rhyme. Pretty tricky stuff, eh? (But ol' Willie The Shakes is long since outta here, and his work is in the public domain, which allows textbook authors to, among other things, bowdlerize it unmercifully.) Courtesy would require that you allow the author to at least know when you're translating his/her work; (s)he might want to choose a translator, or have an expert of his/her choice review the translation to make sure that it's faithful to the intent of the original.
Food for thought is still food, even if it's junk food.