I am a non-native speaker of both English and German, but as a German speaker, I believe that the sentence
"Have you already looked at this excellent textbook?"
is more naturally translated as
"Haben Sie sich dieses großartige Lehrwerk schon angeschaut?"
rather than
"Haben Sie sich schon dieses grossartige Lehrwerk angeschaut?"
The suggested version follows common German word order, where the object typically precedes the discourse particle schon, especially with longer noun phrases.