Some great resources for books are reading apps. If you get on the mailing lists for some of these, you’ll be able to get notified when they have a sale for their annual subscriptions.
Some of these resources may even be free for teachers and accessible for students for free, as well.
We use both of these and some of the resources on the Free Reading Resources, also.
Epic! is offering parents a free 30 days or teachers free remote student access until 6/30/2020. I think I usually wait for a Black Friday offer and its usually heavily discounted for an annual deal.
ReadingIQ is from Age of Learning, Inc. and they also do ABC Mouse and Adventure Academy. ReadingIQ is all about the books and you can get an annual membership at a deep discount right now for $29.99.