This article is from the Pratchett Newsgroups FAQ, by Orin Thomas firstname.lastname@example.org with numerous contributions by others.
Unclear Physics was the WORKING TITLE of the book INTERESTING TIMES.
This particular FAQ writer gets at least three letters a month asking
for the ISBN of this book (which I am sure amuses everyone else on
the newsgroup :-). This happens so often that I got my copy of
IT signed as THIS IS NOT UNCLEAR PHYSICS.
I dunno if it was a joke but Terry recently said that he might
actually write a book called "unclear physics" as the title was
about a particular plot point in IT that never came to fruition.
This is speculation.