"I understand. . . . Actually I believe I know of an expert researcher," said Frode, and before he could stop himself, he added, "She was the one who did the background investigation on you." (p.353)

Slip of the tongue, also know as "the Freudian Slip"

The famous Austrian neurologist, Sigmund Freud, is known as the man who discovered the study of psychoanalysis. He thoroughly pursued this and became very well known due to his extensive work in the area. He coined the well-known term "the Freudian Slip". Basically what this is is when you go to say something but you say something else by accident. 

Ex. Meaning to say, "I'm glad you're here", but saying, "I'm mad you're here".

At this point in the book, Mikael has just uncovered groundbreaking evidence in his search for what happened to Harriet Vanger over forty years ago. He discovered the connection to the initials and names left in Harriet's notebook to murders that happened which are connected with  Biblical passages in the Old Testament. He needs a research assistant with patience to help him go through old newspaper clippings to find more connections to victims. This is when Frode lets slip that Lisbeth Salander had agreed to and carried out an investigative report on Mikael Blomkvist.

Freud says these slips often come from your unconscious and most of the time you are unaware that you are even saying it until it is too late. Frode and Henrik Vanger hired Lisbeth to dig into Mikael's life to make sure that he was the right man for the job and that he did not have any secrets attached to him that could cause issues. Henrik did not want Blomkvist aware of this because he wanted to built up trust and a certain bond between the two of them as they attempted to uncover the truth. Unconsciously though, Frode knew that Salander would be the best candidate for the job and without even thinking he revealed that information to Mikael.