It seems that the person who took Trump's SAT's is named in the book. Also, it's revealed that Trump's sister Maryanne, the ex-federal judge, is the person who did most all of Trump's homework in high school, but of course couldn't take his SAT test for him since she was a girl. So he had to pay one of his male classmates to take the test for him. Note that this was in the days before photo ID's so it wasn't all that hard to pull off.
An excerpt from the book (copies of the book were sent by the publisher to major news outlets earlier today):
Trump “worried that his grade point average, which put him far from the top of his class, would scuttle his efforts to get accepted” at the University of Pennsylvania, where he had “set his sights.” So Trump approached a friend for help.
“To hedge his bets, he enlisted Joe Shapiro, a smart kid with a reputation for being a good test taker, to take his SATs for him,” Mary Trump writes. “Donald, who had never lacked for funds, paid his buddy well.”

OCU