The Spinster’s Retreat by Lateefa Harris

Miss Charlotte Walker has been called many things — forgettable, peculiar, a hopeless case — and that is merely by her own family.
Her mother considers her a walking disappointment, her sisters bully her for sport, and society finds her… a little odd.
Which suits Charlotte perfectly.
After all, it is far easier to remain invisible than risk being hurt for being seen.
Her greatest talent? Hiding.
A skill that proves unexpectedly useful when she overhears a whispered conversation at Lord Bamber’s grand masked ball — one involving murder, a secret society, and two gentlemen with alarmingly casual attitudes towards crime.

Hours later, a man is dead.
And somehow Charlotte becomes the prime suspect.
Now, the most overlooked woman in London has suddenly become the most wanted.

To clear her name, Charlotte must disguise herself as a governess, infiltrate a dangerous web of secrets, unmask a murderous cabal — and somehow avoid falling for the infuriatingly handsome lord who believes she tried to kill him.
Not ideal for a woman who only ever wished to be left alone.