Around December 2015, a little over a year ago, a new book came out published by Robert Siscoe and John Salza. It was entitled:
"True or False Pope? Refuting Sedevacantism and Other Modern Errors"
The book was endorsed and prefaced by Bp. Bernard Fellay, and published by their St. Thomas Aquinas Seminary!
Since then, I read from someone named nαzιanzen on
www.suscipedomine.com that the SSPX regretted endorsing the book.
As well, when this man claimed this a few months ago, I was myself arguing with Siscoe on
www.suscipedomine.com about his major failure in the book about the meaning of "ipso facto", that it was a travesty of error. From that point, he abruptly stopped discussing.
Pretty much since, their website trueorfalsepope.com has had no updates.
Particularly since they saw their error about "ipso facto", I noticed that their book on Amazon was being sold for about $179. This told me that they no longer wanted to sell their book (probably because they had a contract with amazon and then upped the price so that nobody would reasonably pay that price.)
I just looked today and I see on amazon that it has progressed so that the book is now no longer available, saying:
"Currently unavailable. We don't know when or if this item will be back in stock."
It appears to me that Siscoe and Salza realized they failed in their opposition to the sedevacantist position.