Early reader series:
Frog & Toad
Mr. Putter & Tabby

Picture Books:
"My First Little House" books
DS4 loves all the Magic School Bus picture books, but especially the earlier ones, and the chapter books.

Jump Into Science: The Moon
One Giant Leap: The Story of Neil Armstrong

Bad Kitty books

DS4 is currently obsessed with "The Tale of Despereaux: The Graphic Novel"