This website can be used to check the reading level, or Lexile level, of your book. Your Lexile level is on the inside cover of your assignment notebook.
According to Lexile.com, "A Lexile measure is a valuable piece of information about either an individual's reading ability or the difficulty of a text, like a book or magazine article... A Lexile text measure is based on the semantic and syntactic elements of a text. Many other factors affect the relationship between a reader and a book, including its content, the age and interests of the reader, and the design of the actual book. The Lexile text measure is a good starting point in the book-selection process, with these other factors then being considered.”
In other words, when you choose an IRT book, please try to choose something challenging enough for yourself. However, you should also take into consideration the topic of the book and your interests, whether or not it is appropriate for someone your age, and the style of the book.