An accuser identified in court papers as Victim 4 is expected to read the letters into testimony during the trial. Victim 4 will also show the gifts that Sandusky allegedly gave him during their relationship, including a set of golf clubs, an ABC News source said.
A jury of seven women and five men has been chosen. Many have ties to Penn State. There is an engineering administrative assistant at Penn State, a dance teacher in the school's continuing education program and a professor who has on the faculty for 24 years.
One of four alternate jurors was selected Wednesday, and three more were expected to be picked Wednesday afternoon.
Opening statements are scheduled for Monday.
Sandusky, 68, faces 52 criminal counts and potential penalties that could result in an effective life prison sentence. He has denied the allegations.
Defense lawyer Joseph Amendola's potential witness list has seven Sandusky family members on it, including his wife, Dottie, and two sons.
The Associated Press contributed to this report.