The slow presales also raise questions about how strong the ratings will be for ABC's heavily promoted Diane Sawyer ABC News Primetime Special interview with Knox on publication night.
Knox is the Seattle-born American college student who was caught up in the 2007 murder of British roommate Meredith Kercher while studying abroad in Perugia, Italy.
She spent four years in prison before her conviction was overturned for prosecutorial irregularities.
But in early 2013, Italy's high court vacated her exoneration. A hearing will be held later this year on the question of whether she will be retried for the murder.
The case dominated headlines in the U.S. and Britain, spurred on by Knox's photogenic appearance, slightly off behavior (she was seen kissing her boyfriend as they waited for Kercher's body to be removed from the house) and leaks from her diary that painted a picture of her as a promiscuous free spirit.
After her release, publishers and TV networks jockeyed for her story.
HarperCollins paid a reported $4 million for the book.
Every major news anchor pursued her first interview.
The sales and ratings benchmark for Knox and Sawyer is Jaycee Dugard's 2011 memoir A Stolen Life. Dugard spent 18 years in captivity after she was kidnapped when she was 11.
Her book sold more than 175,000 copies on its first day, en route to first-year sales exceeding 1 million copies. Sawyer's two-hour exclusive primetime interview drew more than 15 million viewers.
Waiting to Be Heard has been bouncing between No. 100 and No. 600 on Amazon's sales charts this week.
Given the publicity surrounding the book, including a heavy round of promos on ABC hyping its exclusive publication-night special, reasonable expectations would have seen the book move into the top 50.
Anecdotal evidence also suggests blogs and online news are seeing limited traffic for Knox-related stories.
Early breaks in publisher HarperCollins' strict April 30 embargo also might be hurting sales.
An early New York Times review
by Michiko Kakutani
generally was positive, calling the book "meditative" and Knox "by turns evocative and verbose, sympathetic and enigmatic." But the leaks also suggest the book might be light on the type of salacious details that fueled earlier interest in the case.
Coverage of the Boston Marathon bombings and West Texas explosion also might be hurting Knox. Cable news outlets have focused exclusively on these two stories at a time when they might have devoted coverage to Knox.
It also is possible that viewers exhausted from coverage of those twin tragedies are looking for lighter and more upbeat stories to turn to instead of another murder, especially one whose outcome has been thrown into doubt.
On the other hand, advance requests at libraries are said to be heavy
, which generally is a reliable leading indicator of sales.
The impact of flagging interest in Knox on ratings for ABC's heavily promoted Sawyer Primetime Live interview is uncertain.
The arrival of People's exclusive print interview with Knox on newsstands Friday could change the dynamic.
A further gauge of public interest will be how much of a sales bounce the book gets over the weekend from People.