Choice
Ideas when you here, "I've already read this book!"
Allow students to take the final assessment as a pre-assessment. If they get an 85% or higher, allow them to demonstrate their learning with a different book or activity. (If the pre-assessment scores show the student needs help in a specific area/topic, they probably don’t need a whole unit to learn it, so allow them the opportunity to demonstrate they understand with a different product.)
Or… use the same book, but consider having the student dig deeper by asking them to create questions/activities using Depth and Complexity Icons, Bloom’s Taxonomy, or the Depth of Knowledge Framework.
Or… consider allowing students to choose activities from a choice board