The character of Suzy Bannion (played in Suspiria (1977) by Jessica Harper) is directly referenced as having killed Mater Suspiriorum "many years ago."
The New York residence of Mater Tenebrarum - as seen in the movie Inferno - appears in as a diagram in the book The Three Mothers. Several lines from the same book, first heard at the start of Inferno, are also read out.