Thank you for the additional information, Marlene.
This particular book is published by a granddaughter of Toni Becker's, and all photos, with the exception of one or two, come from her private family archive. Several photos included in the book are not out of copyright yet, and one would need the author's - or author's family's - permission to reproduce them.
Please correct me if I'm mistaken, but I believe the situation is even more complicated in this case. Many pictures in this book are pictures of unique items that have not been published before and are now published for the first time. Then there are several pre-1923 photos that are signed by the Empress or her brother Ernst Ludwig personally, some with a special dedication to Toni Becker: I think they are unique items, now lawfully published for the first time too. A person - in this case the author - who, for the first time lawfully publishes or lawfully communicates to the public a previously unpublished work, benefits from a protection equivalent to the economic rights of the creator, and the term of this protection is 25 years from the time when the work was first lawfully published or lawfully communicated to the public. Wouldn't this provision protect the great majority of pictures in this particular book for the next 25 years?