Here are some suggestions that you can offer to parents/kids to help them find "just right" books:
Strategies to help parents and children decide if a book is "just right"
Browse websites such as Amazon, Goodreads, and BiblioNasium. Type in a book you remember loving and that felt "just right". See what recommendations pop up.
Search by guided reading level on Scholastic
Level It Books - app for Apple and Android devices. Scan the book's ISBN to find the Lexile, Guided Reading Level or grade equivalent.
The Lexile Framework for Reading. Type in a Lexile Level to access book lists at that level.
A leveled book list separated by focus (fiction, nonfiction, content studies) from Arizona State University
Teachers College Reading and Writing Project Book Lists - The overall organization of these lists is by genre, type, or theme. You have to scroll all the way to the right to find the Guided Reading level or recommended grade level.