4th Down by Kate Calloway

Under different circumstances, a week at a rustic lesbian retreat would undoubtedly bring much-needed rest and tranquility. But unless she can track down a ruthless killer, Cassidy James may be headed for another sort of peace and quiet -- the kind that comes with headstones!
Someone is trying to kill Dr. Allison Crane, head physician at a Portland women's clinic and president of Women On Top, the influential lesbian organization to which she has willed her substantial estate. Reviewing the evidence, Private Investigator Cassidy James quickly deduces that the culprit is someone dose to Allison -- someone who knows the intimate details of her daily life.

Posing as Allison's new girlfriend, Cassidy accompanies her to a Women On Top retreat deep in the Oregon wilderness, hoping to lure the would-be murderer into the open. With danger lurking just outside their cabin, the undercover detective discovers that the alluring Allison wants her under-the-sheets as well -- and the feeling is overwhelmingly mutual....