The Multifaceted Mind: Book One of the Many Minds Mysteries (The Many Minds Mysteries Series 1) by J. Traveler Pelton

Allyson the gatekeeper was a high functioning multiple personality raised in a loving home by professionals. Her various egos had their part in the family firm, from accountant to detective, from gardener to infant, from martial arts master to shy nerd; so many voices speaking at once with the gatekeeper to get their time in the sun. When a co-worker is found murdered in her garden, she must find a way to clear one of them of a murder charge. If you want to move beyond plain mysteries and into the just plain weird but possible tale, you have to get the Multifaceted Mind