Frog and Toad.
I've found that both my kids are/were pretty sensitive to the reading level until they hit about the mid-third grade level. I have used the scholastic book wizard (
http://bookwizard.scholastic.com) to find books at the same level as ones that are successes. Henry and Mudge are about the same level as Frog and Toad at DRA 18-20 (early second grade-ish). Cam Jansen is a wee bit higher (20-24), and Dinosaur Cove & My Father's Dragon are higher still (DRA ~ 30).
My kindergarten DS is at about the same level, and progressing at a nearly identical rate and time line. He's mid way through the Frog and Toad series. We also like the set of Rookie Read-About books by Allan Fowler right now. I found them with the Book Wizard as well.
