A US perspective, with some reviews.
The other UK-based harmonica dealer, with a comprehensive selection of spares (including new combs for the older chromatics I've got).
This is accurate for the SU-21, at least. The web could really use a comprehensive list of harmonica tunings, though.
Shipped next day. (My neighbours must hate me; this is probably more annoying than the banjo.)
Not for complete beginners (the question I was trying to answer was "which hand goes at the top"...), but lots of interesting stuff.
"How to play the jew's harp in a virtuoso manner: Instructions and sound examples". Good analysis of how it works and why it sounds that way (and a number of tricks I'd not been taught before).
More detailed instructions. Some of this is terrifying.
Good descriptions of how to do basic stuff from a luthier. (Or whatever an accordion-servicer is called...)