I haven't ever read any of them, but I've heard nothing but good things about the Magic Tree House books. My 5 year old daughter loves the Clementine books by Sara Pennypacker and she giggles nonstop while I read to her.
Other good choices might be:
Mrs. Piggle-Wiggle books
Captain Underpants books
Roscoe Riley Rules books
George Brown, Class Clown books
Horrible Harry books
Nate the Great books
Cam Jansen books
Harriet the Spy books
The Boxcar Children books
A to Z Mysteries books
My Weird School books
Wayside School books
Judy Moody & Stink books
Junie B. Jones books
Charlotte's Web
Beverly Cleary books
Roald Dahl books
A great place to research and get more ideas is Amazon.com.