I have all of them. All in paperback. ALL worth reading. :} 'Someone is watching' is really the only one I had trouble getting a hold of. I had to hunt for that one. Some told me It was out of print, others said they had to order it from other stores on the other side of the country. Borders finally got it for me. They're pretty good about getting anything you want. Barns and Noble is great(discount and all) but if they can't get one of Mark's books then Borders is your best bet.
P.S. If your not sure what to read next, I suggest going in order. They're all listed in order on the site. Just don't forget 'The Vampire's Heart' For some reason it's not listed here anymore. ??? (It's basically last, kind of on it's own)

I have them all as well. You can get any of them off of Amazon.com. I would definitely recommend them all. You should try to get them in order because in each preceeding novel there are little connections to a previous book.