You've gotta love the Interweb. After putting in a search for the words Stella and Marni, what should Google pop up but a book entitled Assignment Stella Marni. Written in 1957 (the year I was born!!!), it seems to be a steamy private eye novel centered around a dangerous blond who was "murder on heels".
What an strange coincidence.
I've purchased a couple of copies of the old paperback. A nice oddity to have around. Although I don't think it will be one of the books we read to the girls each night.