A very interesting premise and one that Lovecraft is known to frequently dabble in, that of the Evil Wizard. Unlike the regal but nefarious Saruman, Lovecraft's evil wizards are always nasty, crusty, twisted people. The entire triggering incident is caused by infidelity of all things and ends with 3 people and a dog turned to stone.

I am getting a tad tired of the "convenient book/journal that explains everything" trope. It's just a lazy way to explain what happened in your story after the fact. There are better ways to present exposition, thank God.

I also kind of hate that the wife committed suicide. It just feels like robbing her of all agency. She fights for so long to be free and when she has the opportunity to be, she chooses to be a statue like some dude she's known for 4 months. Just kinda shitty.