Renaissance - E. H. Lupton

June, 1971. As the academic year draws to a close, Sam and Ulysses are looking forward to a quiet summer. But when Ulysses's grandmother is hospitalized, it becomes clear that relaxation is not in the cards. Unable to accept that her fall was an accident, Ulysses begins to investigate whether it may be related to a cult from their past whose mysterious and powerful leader seems to be popping up all over town.

Sam's doing his best to hold things together and be supportive, but it's hard when his new husband is barely listening and keeping him at arm's length every time family is concerned. And on top of everything else, the library has something urgent to tell Sam...

As they reunite with old friends and prepare to bid farewell to others, Sam and Ulysses will see their marriage tried, their lives threatened, and meet an old enemy they thought long dead.

Renaissance is the fifth book in the Wisconsin Gothic series. It would probably be best to start with book 1, Dionysus in Wisconsin.