A series of e-books featuring the earlier years of the cartoon have been made available for distribution via the WOWIO e-book service. The deal pretty much can't be beat. The books are free, and there's much more to choose from apart from "Day By Day" books.
The WOWIO folks are keen about obtaining free promotion, as they encourage links like the one above.
Here's a third party review of the service for those who (like me) are suspicious of free stuff.