Knowing what your child is comfortable reading is important for choosing a reading list that suits the child's ability. For me, it was important that my children feel good about their reading skills and at the same time meet some challenges.
The Lexile Framework is a great tool for selecting books that are progressive, or within a certain ability range. Other leveling systems exist out there, like the grade level system found on the backs of some school age books. For example 4.2 indicates 4th grade 2nd month. And there's the letter system used by school teachers that begins with AA and goes to Z. These are great if you are looking at a series of books which are already catalogued. But what about the great literature out there that hasn't been catalogued?
Lexile Framework has been a great source of reference for us.
For example here are the results for 20,000 leagues under the Sea:
Jules Verne's original: 870L
Retold by Carl Bowen: 440L
Judith Conaway's adaptation: 440L
This is a very helpful chart for determining which book is appropriate for my children to read.
My eldest is reading in a lexile range of 700-900, so the original is fine for him. But, if a younger sibling wanted to read the same story, either of the 2 retellings would be more appropriate reading level.
Try a search for yourself and see!