I noticed the references to mathstart on this thread, and just wanted to add a link from their website, which even includes a list of other author's books. Here's the link:
http://www.mathstart.net/books/math_bibliography.php. When you click on a mathstart book, it provides great little ideas for activities that you can do with your child to reinforce what they're learning.
A few quick tips that you may not realize if you're just getting started:
1. you can search these books on amazon and take a peek inside to see what they're about
2. you can have your library do an interlibrary loan (or even request for them to purchase the books) if they don't already have them available
3. don't underestimate youtube ... brilliant source for catchy math videos
Enjoy!