Another series DD would have loved when she was on her book binge at age 3:

http://www.amazon.com/Mouse-Mole-Perfect-Halloween-Story/dp/0547551525

The tone is really sweet, much like Frog & Toad but it's probably slightly harder to read than F & T.

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.

my DS loved reading "little critter" books at 3.

Mouse and Mole series by Wong Herbert Yee

Ant and Honey Bee by Megan McDonald

The latest Piggie and General book is a must-have:

http://www.amazon.com/Waiting-Easy-Eleph...waiting+is+hard

I just discovered an early reader series called "Catwings" that looks spot on for this age bracket.

Geography from A to Z, J. Knowlton
Little Elephant, Big City, M. Curato
Voices in the Park, A. Browne
The Mystery Hat, Bennicke & Jensen
The Way We Work, D. Macauley
The Firefly Spanish/English Dictionary (excellent visual dictionary)
What's Science all about? Usborne
Greg's Microscope, M. Selsam
How Mountains are Made, K.W. Zoehfeld

Chapter Books:
DS5 is really enjoying the "Shark School" books by "Davy Ocean". I think the first book in the series is Deep Sea Disaster, followed by Lights! Camera! Hammerhead! We were able to get these 2 at our local Barnes & Noble, but it looks like we'll have to order subsequent ones online. Main character is a hammerhead shark.

Exciting fiction news!!! The Roald Dahl novels are now available as unabridged picture books, with colour pictures on each page of text! DS3 and I just read "The Fantastic Mr Fox", and he adored it! They're perfect for children who crave a more involved plot, but who still prefer pictures. It took 90 minutes to read the book start to finish, complete with wiggling, bouncing, and speculation.

http://m.indigo.ca/product/books/fantast...fox&ikwsec=Home

Fantastic - my 4yo old still greatly prefers books will color illustrations, but needs more challenging text.

I've been looking at many of these books and they look like books my daughter (4) can fly through in a matter of minutes. I'm a little at a loss for her because I don't want to provide her with too mature of subject material but I'd like to provide her a variety of books - easier and more challenging. I got her a Ramona book and she seems mildly interested, and she has no issue reading it - but she really is more drawn to books with pictures still. She has also read some American Girl, which I think are fairly safe for her. I'm just boggled because her reading is so advanced past her age, I don't know where to point her.