Since I oversee the library's collection of children's books (here primarily to support our future teachers in the College of Education), I sometimes get asked for recommendations for books at certain reading or grade levels. Sometimes this is because the child's school participates in Accelerated Reader; sometimes it's just because someone is looking for a book appropriate for a particular grade.
I won't go into the pros and cons of Accelerated Reader and other leveling systems here, except to say that if the child WANTS to read a book above grade or reading level, I certainly would not discourage the child from trying! These systems are just guidelines. Keep in mind too that even if the reading level of a book measures out as low, the content of the book may be more appropriate for an older child; and vice-versa.
If you want to look up Accelerated Reader information yourself, you can do so at this site: http://www.renlearn.com/store/quiz_home.asp. Keep in mind that not all books have an AR quiz (and thus won't be assigned a grade level), and that even if the book HAS a quiz, the child's school may not own that quiz. This page will also give you the interest level for books (UG = grades 9-12, MG = grades 4-8, LG = grades K-3), which can be very different from the reading level!
Another system for measuring the reading levels of books is the Lexile Framework. Lexiles are reported with the state's Texas Assessment of Knowledge and Skills (TAKS) test scores.You can look up the Lexile measure of books here: http://lexile.com/fab/, while this page explains how those scores roughly correspond with grade levels: http://www.lexile.com/about-lexile/grade-equivalent/grade-equivalent-chart/.