DORTMUND, GERMANY - APRIL 11: (EDITORS NOTE: Image has been digitally enhanced.) Ole Book (R) and Head Coach Niko Kovac (L) after the Bundesliga match between Borussia Dortmund and Bayer 04 Leverkusen at Signal Iduna Park on April 11, 2026 in Dortmund, Germany. (Photo by Lukas Schulze/Bundesliga/Bundesliga Collection via Getty Images)

Book (l.) and Kovač (r.) will work closely over the summer.