Why are some Photo books now going for so much money?

Edge of Darkness by Barry Thornton - I find most of the content just common sense.

The Master Photographer's Lith Printing Course by Tim Rudman - Most of the photo materials/supplies that are noted in the book are no longer available.

The Photographer's Toning Book by Tim Rudman - Again most of the photo materials/supplies that are noted in the book are no longer available, though the chapter on Formulary I have found to be useful but the info is readily available on the WEB or in other books.

THE PLATINOTYPE 1977 by Nancy Rexroth - 30 pages of info that is covered at extended depth in other books.

Bought these books (and others) when they first came out, used them over the years, but now very rarely refer to them. I've seen the above books (and others) selling for exorbitant prices lately. Prices reflective of demand? Just curious...