Generally speaking, books in "Learn to Read" and "It's Fun to Read" are DRA level 1. Books in "I'm Reading" range from DRA level 8-16. Book sets from the Margaret Hillert "I'm Reading!" series are DRA level 8.
Visit The Starfall Store, select "Reading" then "Reading Levels" for a complete listing.
You may also find Starfall Book Levels at the Teachers' Lounge.