4 of my books were adjacent, and when they destroyed themselves no bookshelves were made :(
Viewing post in Million Monster Militia comments
Happened to me too.
It appears to happen anytime 2 or more adjacent books are each next to 2 books themselves, as they all destroy each other before one can transform.
A couple possible configurations where this can happen (B = book, X = Not a book):
| X | B | X |
| X | B | B |
------------
| B | B | B |
| X | X | B |
------------
| B | B | X |
| B | B | X |
Etc etc