The Argon Legacy

After barely surviving a US Army contract turned hostile in Vietnam, Charles Albright has settled into a quiet life in Hawaii as a realtor. But that changes when a letter arrives notifying him of a mysterious inheritance from his aunt Helga, stowed in a safe–deposit box in Philadelphia. As she was penniless at the time of her death, he can't imagine what could be in it or why she left it to him and not her own children.

-- J. Albert Griffiths