Quite by accident I came across CK12 http://www.ck12.org/teacher/. These look like very nice resources. I haven’t read through much in detail, but they have FlexBooks which are very nicely put together. The bit I liked most is that they have online interactive worksheets which I could just work through. They are really aimed at the higher end of the US school system (K12), but I think there is material in here that will be really widely used.