Understanding that
- there are various systems for determining reading levels,
- various passages of books could have different reading levels,
- the specific application/use of reading levels in education may be flawed...

... I am also familiar with corporate environments which require all documents produced to be within a prescribed range of reading levels...

... and I think it would be GREAT if more / many / most books and/or online resources displayed their reading level, or reading level range.

Although the specific formula developed/used by some organizations may be proprietary, reading level ranges and readability index which I am familiar with are determined by some combination of:
- words per sentence,
- syllables per word,
- syllables per sentence.

Individuals can assess reading levels of materials using online tools.

Reading books of a certain level and/or assessing the reading level of a number of books, could be goals which one sets for themselves in working toward achieving a portion of the requirements in earning the Congressional Award for youth (for which the cutoff is the individual's 24th birthday).