"D-Day Through German Eyes" by Holger Eckhertz.

It is a series of interviews done by a German military journalist during the war, and spent the postwar years interviewing fellow veterans of the Normandy campaign. The interviews were kept in the family and recently published by the grandson.

The interviews show a lot of variety, but some common themes include the shock many defenders felt when they saw the mass of ships and aircraft on the horizon. They had been anticipating an invasion, but the sheer scale was hard for the mind to grasp. They also frequently complained that the western Allies were invading at all, and that the local French treacherously betrayed them through sabotage.

Volumes one and two are both widely available, in print and electronic, and sometimes even for free through certain sites shared libraries. So this is maybe the most accessible and engaging source on the subject, without going into archives.