Note that "a" Flat Stanley (one of the books in the series) is different from "the" Flat Stanley (the original, unabridged book on which the series is based).
Accelerated Reader (arbookfind.com) says the books in the series are 2.2, 2.3, 2.4, or 3.3. The original is 4.0 unabridged and 3.2 abridged. Roald Dahl's books are mostly 4th grade level; the Narnia books are mostly 5th grade level.
Book leveling is an art (judgement is required), but the way books are actually leveled is a science (formulas are mechanically applied). I don't think you're going to find a satisfactory answer to your questions because of that mismatch.
Accelerated Reader quizzes mostly test recall. There's none of them that care about pronunciation - that's tested separately as a reading fluency test (our district uses DIBELS). STAR Reading (
http://www.renlearn.com/sr/skills.aspx), which is what our school uses to assess reading level, does test comprehension. The reading level you get is basically "your score is at the 50th percentile for kids of that grade-and-month who were in the norming sample."
There is enormous spread in reading ability among younger kids. My DD's first grade teacher said that she had kids who tested anywhere from kindergarten to 6th grade level at the end of first.
