Initially, this book does not appear to be in the library, and Grace concludes that there are no books in English that she can read about Ludwig II. Only after Grace is contacted through her dream by Ludwig II does she see the book. It's unclear whether the book was actually there, and Ludwig merely prompted Grace to see it, or whether it was placed there by supernatural means. (A close look at the video shows that Grace is not looking at the same bookshelf at all, indicating either a continuity error or that Ludwig books were placed on more than one shelf.)
The author of the book had connections in the German government and was given special access to Ludwig II's diary, on the condition that he never publish it. Chaphill concludes from the entries that Ludwig was driven mad by the conflict between his catholicism and his sexual orientation.
Chaphill wrote an entire English translation of the diary. After his death, this passed to his son, Thomas. Thomas initially will not give Grace a copy of the diary. With help from Ludwig's spirit, he relents and faxes a copy of the diary to Rittersberg.