Heritage History has taken many old books that are no longer copyright protected and converted them to an online format for easy access. They even have color pictures included. They cover many different civilizations and offer other features with their website like maps, the ability to ask question and read responses, and also their advice on history teaching. I think this could be a huge part of any history education and it is free.
I hope you will find this resource useful. I have also compiled other online book websites on my Free Online Books page.