When reading, it is important for your child to be reading at his or her "just right" reading level.
We will be using the Fountas & Pinnell reading levels, which are sometimes called Guided Reading levels (if there is a choice, use F&P). These are written as a LETTER, NOT A NUMBER!
Scholastic has a tool called Book Wizard which has many books that can be searched and leveled. This is the link. Unfortunately, they sometimes list a number or grade level, not a level. This link provides a really great Text Level Correlation Chart. By using that chart, you can easily convert a number to a Fountas & Pinnell letter.
For example, if the Book Wizard shows that a book has a Grade Level Equivilant of 4.3, but doesn't list a letter, then go to the Text Level Correlation Chart. On there, a 4.3 in Grade Level roughly correlates to a Q in Fountas & Pinnell.
I expect that the class should be reading at their grade level- or possibly 1 level above or below. Anything different needs my approval. If a book is much higher and they want to continue to read it, then it would make a wonderful book for you to read and discuss with your child.
If a book level is not attainable through the websites above, which is likely for older books and books that aren't as well known, then I suggest having your child do the 5 Finger Rule. They should be familiar with the concept of reading 1 page in their book. If they find 0 unknown words, it is too easy. If they find 5 unknown words on one page, then it is too hard. Anything in between should be fine. See below.
I know it might be difficult for them to break from reading books that are way above their level such as Harry Potter or Dork Diaries, but in order for them to get the most out of what they are reading, they need to be reading books that they can read fluently and comprehend as well. More difficult books have features that they might not be able to comprehend, which hinders their reading of the book. Conversely, if they are reading books that are too easy for them, they are not getting anything out of the book aside from enjoyment, which is great some of the time, but not during the assigned homework time.
Please let me know of any questions and thank you for helping your child get the most out of their very important independent reading time!