I'd agree with Sorcha - the "Wee Sing" series of songbooks/recordings are excellent.
I also like to collect grammar school music textbooks. I get them at library book sales for a buck or less, and some are excellent.

There are several out-of-print Fireside songbooks that are very good, with nice illustrations. If you go to used booksellers like www.bookfinder.com, enter fireside songs and a nice selection should turn up.

-Joe Offer-