A Marvellous Light by Freya Marske (The Last Binding #1)

Set in an alternative Edwardian England, this is a comedy of manners, manor houses, and hedge mazes - including a magic-infused murder mystery and a delightful queer romance.

Young baronet Robin Blyth thought he was taking up a minor governmental post. However, he's actually been appointed parliamentary liaison to a secret magical society. If it weren't for this administrative error, he'd never have discovered the incredible magic underlying his world. Cursed by mysterious attackers and plagued by visions, Robin becomes determined to drag answers from his missing predecessor - but he'll need the help of Edwin Courcey, his hostile magical-society counterpart.

Unwillingly thrown together, Robin and Edwin will discover a plot that threatens every magician in the British Isles.