You may find your poem still tends to reflect themes in the book, an interesting consideration for authors on word choice. Alternatively your poem may turn out to say something other, deeper, darker. You may also find as I did, to my utter surprise, that even in found poetry, not everyone gets it; there are poets out there even if supplied with others' words, and people who see the world, perhaps, in a more concrete or less verbal fashion, and may struggle a little with the concept. An alternative assignment, rather than asking your poets to choose words, is to give a passage from which they redact words, black out words with a marker, and then read what words are left. This also makes something visual to look at, with the black stripes, so has an appeal for the more visual art oriented.
Please remember to attribute the original author. I made mine with words from Allison Mills's The Ghost Collector, a juvenile chapter book. I think a found poem is a nice review of, or plug for, a book.
Water, heavy, dirt.
Drizzling, gray, sad.
I don't understand what happened--
Do you know my name?