Reading A-Z is a great site of reading resources. Using leveled readers, you can assess your child's reading level. If you click on the Assessment tab at the top of the page it gives you complete instructions on how to properly assess their reading. This format is very similar to what is used in schools. There are a large selection of books available which could be printed or read from the screen.
At the bottom of the page is a list of associated websites; Science A-Z, Writing A-Z, etc... I haven't checked them all out but the reading site is definitely worth while just for the selection of books. The only problem with this website is that there is so much there that it takes a little time to navigate the site and find what you are looking for. Definitely worth the effort. Enjoy!