Some popular titles for that age range age range (8-9 year olds, 3-4 graders) are:
Diary of Wimpy Kid novels (a new one, The Third Wheel, just came out; more boy focused)
The Dork Diaries (more girl focused)
The 39 Clues series
The "Origami" Star Wars series (Strange Case of the Origami Yoda, etc.)
The above are all chapter books, but you might also consider books like The Essential Pokemon Handbook, or the Lego Character Encyclopedias (Star Wars, Ninjago, Super Heroes). The books are colorful and have lots of fun "facts", and don't require the commitment or time that a longer chapter book does. They are even fun to just look through when you don't feel like reading.
The DK encyclopedia series is also popular. Volumes focused on everything from ballet to weather to favorite animals with great full color photographs and interesting information.
Kids around this age also love cartoon collections. Our school library can't keep Garfield, Calvin and Hobbs, Family Circle, Peanuts Collections, etc. on the shelves.