Yes! For starters, Group 15, the group of assassins that John Milton was originally a part of, shows up in most of the books as either antagonists or protagonists, and they play a massive role in the vignette series “Group Fifteen.” Additionally Milton and Micheal Pope (from Isbella Rose) cross over in the book Sleepers and Milton and Beatrix Rose crossover in the novella Phoenix. Finally, since they are mother and daughter, Beatrix and Isabella Rose often appear together