In a recent post, I mentioned my re-acquaintance with my childhood as a result of an RMIT course.
There is another side to this acquaintance because the books I read as a child are not out of print. Because of my obsession with old county cricket, ever sicne I realised how poorly maintained the State Library's old Wisdens are (though they are a little better now than they were in 1991 when my old county cricket obsession began) I have always wanted to have my own copy of these ancient books. As a result, I know very well, especially as a result of my contact with Yallambie book merchant Roger Page, the collectibility almost all Wisdens except the most modern and least interesting.
Because of this, when I had to look at Time Machine books for my RMIT course, I had to know how much they would cost for sale on eBay. Being as obsessed with statistics as I am, I naturally tried to find out which were the hardest to find - the hardest book in the series to find was Mission to World War II, owing to the collectibility of books on Nazi Germany.
I spent the whole of today at home trying to look at the collectibility of Choose Your Own Adventure books. I have excluded those authors whose books have been reprinted since 2005 because the same stories are still available even if modified. Demian told me that it would be the later books that were most collectible because they were never reprinted. My research, based on experience at certain outer suburban bookshop and thrift shops and on eBay over the last month, shows that Demian is basically right. However, the chart of my research shows that titles between #54 (Forest of Fear) and #70 (Invaders of the Planet Earth) are almost as rare. I have tended to presume this to be because of several authors who never wrote a book for the series at a later date having their books removed from printing quite early in the process and fewer copies having survived than of titles between #71 and #140. Authors of whom this might be true include Meryl Siegman, Ann Hodgman, Deborah Lerme Goodman and Tony Koltz (his Terror Island I have seen sold only by one buy-it-now store on eBay, but Vampire Express is far better-known and was reprinted until the series ended)
Another notable feature is the "You Are..." books tending to be harder to find than others of similar number. You Are A Shark, the first book of this type, is also the only among the first hundred I have not found absolutely anywhere. Later "You Are..." titles are similarly rare.