Because the publisher went out of business, it has become harder to find the books (though inventory does still exist). They are not sold through this website but you may be able to find them at your local fly shop or online (though no longer through Amazon).
A couple of places that I know, for sure, carry the books on a regular basis are:
If your local shop does not carry them, please ask them to order books through their distributor.
If you still cannot find the books, contact me.