The Monstrumologist by Rick Yancey

Goodreads Storygraph

"These are the secrets I have kept. This is the trust I never betrayed. But he is dead now and has been for more than forty years, the one who gave me his trust, the one for whom I kept these secrets. The one who saved me . . . and the one who cursed me. So starts the diary of Will Henry, orphaned assistant to Dr. Pellinore Warthorpe, a man with a most unusual specialty: monstrumology, the study of monsters. In his time with the doctor, Will has met many a mysterious late-night visitor, and seen things he never imagined were real. But when a grave robber comes calling in the middle of the night with a gruesome find, he brings with him their most deadly case yet."

...and so, The Monstrumologist? Ok, unclever series name aside, this is a genuinely great period horror series about a traumatized young boy being raised by a mad scientist. It's well-written and emotionally fraught, and the horror elements are thoughtfully considered. If you're obsessed with John and his fucked up (pseudo-)paternal relationships with Harrow and Gideon, I need you to read this.