The German giants are now pursuing younger alternatives including Arsenal winger Ethan Nwaneri and Hertha Berlin prodigy Kennet Eichhorn.
Sancho is available as a free agent following the expiry of his Manchester United contract and has been linked with a return to Borussia Dortmund, the club where he won two Bundesliga titles between 2017 and 2021.
According to Sky Sport Germany, he has not been ruled out entirely and could resurface as a target later in the window if no other deal materialises, but he is no longer Book's immediate focus.
Nwaneri, the 19-year-old English under-21 international, has been on loan at Olympique de Marseille this season and is understood to be open to joining Dortmund. He contributed two goals and one assist across 11 appearances in France and returns to Arsenal this summer with his future unresolved. Arsenal value him at €40m to €45m and are reluctant to sanction another loan, which makes a permanent deal considerably more complicated for a club operating within tight financial constraints.
Book has been direct about his admiration for Eichhorn, the 16-year-old Hertha Berlin central midfielder who is among the most sought-after teenage prospects in European football this summer.
"I do think he has a certain creativity about him. We know him well, and we like him. Like probably many other clubs, we like the player. We'll have to wait and see what happens next."
Manchester City and Bayern Munich are understood to be ahead of Dortmund in the race for Eichhorn, whose release clause is worth approximately €10m to €12m — though the total package including wages would represent a considerable commitment for a player of his age. Atletico Nacional's 17-year-old forward Samuel Martínez, also attracting interest from several English clubs, is also on Dortmund's radar.