New England Patriots acquire a new quarterback due to uncertainty around Matt Corral.
Book, 25, made a mark in college football at Notre Dame.
Book started a game for the Saints in the same season due to quarterback illnesses, resulting in a 20-3 loss to the Dolphins.
After a stint with the Eagles' organization last season, Book was waived in late August.
The Patriots are adding him to bolster their quarterback depth.
In his sole NFL appearance, Book completed 12 of 20 passes for 135 yards with two interceptions.
He is set to be the third quarterback on the depth chart, behind Mac Jones and Bailey Zappe.
Book's eligibility to play in games depends on a promotion from the practice squad to the active roster.