Your child can finish a short book, but a thick one still goes straight back on the shelf. This trail builds the muscle for the thick ones. Start at the top with the shortest, and each book is a little longer and asks a little more staying power than the one before, so by the end a full-length novel feels like something they can actually get to the end of, because they've been quietly training for it the whole way up.
What grows from the first book to the last is simply how many pages your child has to keep going for. The Chocolate Touch is short, funny, and over almost before it starts; by the top, The One and Only Ivan is a real 300-page novel. Start at the beginning and let each rung feel easy before you climb to the next, because every book was picked to be only a little bigger than the one before it. The second rung, Sarah, Plain and Tall, is actually the shortest book here — it steps the reading level up a notch while keeping the page count tiny, which is a gentle way to take that step. Nobody has to skip ahead, and there's no prize for rushing. Finishing the last one is the whole point.