I have my kids read challenging or instructional level for formal reading, use their comfort reading level and their maturity level to pick readings for other subjects and allow them to pick what they want for free reading, even if it is See,spot. For my ds12, this doesn't always work since he is reading at an adult level, but I am not comfortable with the content on most adult level reading, so even his formal reading is slightly lower than his challenge level. I concentrate on broadening him, adding biographies, history, science, etc. to stories. For my ds7, Green Eggs and Ham is his challenge level, so I read aloud a lot to him, because he is mature enough for higher material, just doesn't have the skill.
There, clear as mud. And I also reread many of the kids books as I assign them. They remain favorites over generations for a reason.