Take pictures of every page before you touch anything else, lol. Then hit up your local historical society, but be ready for some pushback if it messes with the town's favorite story. Make digital copies before you let anyone else hold that thing, trust me. It's wild how often the real story gets buried under the simpler version people want to believe.
Expect some local historians to dismiss your find right away. Their whole image might depend on the story everyone knows. Stick to your guns with the diary as proof but pick your battles carefully.