Scholastic has a Teacher Express store where you can buy teacher's resource books in eBook format. The books usually range from $5 to $25 dollars each. They have books targeted to preschool all the way to books targeting high school. They cover a wide variety of subject areas as well.
Through September 20, 2011 they have 849 of their eBooks on sale for $1 each. I think it will combine with the coupon code 10THANKS for $10 off your order. You can actually choose $10 books for free without entering any payment information at all and immediately download the books in pdf format. You do need to create an account with them.
I haven't actually placed my order yet, but I have 45 books in my cart so far. I may need to narrow that list down a bit before actually placing my order. They seem to do this sale once a year. Last year I bought 20 or so books and have enjoyed them.