The Secret Keeper - A Dick Hardesty Mystery, #13

P. I. Dick Hardesty listens with polite interest to his partner Jonathan's stories of his days working for 90-year-old multimillionaire Clarence Bement, helping the old man tend his garden. But when Bement is found dead, an apparent suicide, Jonathan is adamant that the old man would never have killed himself, a theory also held by Bement's grandson, Mel Fowler. When Mel hires him to investigate, Dick learns Bement's lawyer also died mysteriously, barely a week before Bement.
Dick finds himself immersed in a world of greed and familial dysfunction, searching for a missing new will, and Jonathan becomes the target for someone who believes the old man entrusted Jonathan with a secret he is not aware he has.