Years ago someone suggested making a book out of the Challenge entries -- I think from memory the idea was floated twice -- but it's something that really isn't going to get off the ground for practical reasons, and I recall Brian was also a bit put out at the idea, since it might have been seen as taking from/ripping off Chrons (in a moral sense, if not legal).
As to the issues involved which would have to be considered, first, there's copyright -- the stories can't be lifted straight from the Challenges, as the authors need to agree to allow their work to be published elsewhere, and getting consent from everyone would be impossible since a lot of members have left and don't return; indeed, some can't return. Then there's the issue of an editor -- ie someone who decides which stories are suitable for inclusion -- are you looking for a premium product, so it's only the best stories which are picked, or is it a free for all and quality doesn't count? And simple editing -- for the Challenges we can't change the stories once the editing window is closed, but would you to allow them to be tweaked to add the missing comma, or correct the mis-spelling, or larger changes prompted by comments in the Improving threads? Then there's cost and admin -- who is going to do all the work of getting it together, designing the cover, laying it out, making the index and everything else involved? Most importantly, who on earth is going to buy a book, when they can get all the stories for free and/or any prompt by reading them on Chrons? And in the unlikely event it is bought, who gets the money?!
Having said that, we each of us retain copyright in our own stories, so if anyone wants to try it with his/her own stories and those of friends who give their consent, the option is there, and other prompts could be included, and perhaps stories written specifically for the book if the numbers need to be made up. I've never seen a writing prompt book, but if they're popular, perhaps someone is making money out of them. What to me seems more likely, though, is that they might act as effective give-aways for people with other ventures they're trying to promote, ie websites, blogs, novels.
Good luck if anyone wants to try it.