There are some great ones already mentioned in this thread! DS4.5 has just started the Captain Underpants series (about which I am feeling conflicted - the indulgence in rip roaring potty humor is worth the laughter and confidence he's demonstrating). He also enjoyed Eerie Elementary (a good one for Halloween holiday fun). Over the past 2 years he's much enjoyed these too:
many of the Magic Tree House books
Roald Dahl (especially James and the Giant Peach) read together
Mr. Men and Miss books (Mr. Impossible, Mr. Forgetful...)
Magic School Bus books
Dodson series
Poppleton books
Mercy Watson series
Frog and Toad series
Mr. Putter and Tabby books
Oliver Jeffers books
Danny and the Dinosaur
Judy Viorst's Alexander books
Harold and the Purple crayon books
Morris the Moose books
Fly Guy ;)books
and Elephant and Piggie books - one of which was his first independently read. I'll never forget the laughter while he was reading it or the happiness he felt talking about it afterwards. He still pulls them out every once and awhile.
