My correspondence school (educationdirect.com) used the Mike Holt book on estimation.
It seemed complete (as complete as any book on estimating can be!
) and his site seems pretty thorough.
My only concern would be accreditation - some AHJ's only accept accredited schhols for distance learning - and I'm not sure if his stuff is. Look for the approval of the US armed forces - it's usually a good "no BS" indicator.