That Sweet Little Old Lady by Randall Garrett audiobook.
Randall Garrett had this story first published in Astounding Science Fiction September and October of 1959. His twisted sense of humor and gift for the bizarre situation with believable characters shines here. In the not too distant future, Ken Malone, young but promising FBI agent , is given the most important and difficult assignment of his career: find a spy who is stealing information from the Ultra Top Absolute Secret project to develop a non-rocket space ship at Yucca Flats Labs in Nevada. But this is not a normal spy, this spy laughs at the FBI and all attempts to find him or her because they use an unknown new method to steal the information directly from the minds of the scientists. And then of course, there is the Sweet Little Old Lady who is immortal and the stunning nurse and the Las Vegas gangsters and ... well, you had better listen to the story to find out the details