Also known as 'The Walker by the Lake', this book was written by Alexander Roby and published, in 1923, by Whitehall Press, a vanity publisher. The book is bound in all-white cloth, blank cover and spine. The title page gives the title, in German and English, the author's initials 'A R' and a quote from Coleridge - Suppression Prepares for Overflow.

Neither narrative, nor poetry, nor drama, the book is nonetheless fascinating reading, with the text is split into halves headed 'First Act' and 'Second Act' and describes the dreams of an unnamed man, in the form of a case history. The dreams themselves are all recounted in German, though the rest of the text is in English. It seems a fair guess that the book is in some way autobiographical.

The dreams relate to a power that the dreamer slowly comes to know, a power he refers to as 'The King', 'Kaiwan' or 'the Unspeakable One'. Eventually he visits the city of Carcosa, a place of beauty and contentment.

Der Wanderer durch den See was a key driver of the party's early investigations:

• Doctor Lionel Trollope was reading from this book - Roby reciting the text from memory alongside him - when he had a vision of his own death, a vision that proved all too accurate.
• Doctor Bartlett tasked Nathaniel Browne with finding a copy pf this book, although the party was given a copy of the book by Delia Morrison before he could source one.
• Browne eagerly read the book once he received it, and while he instinctively knew that it held a deeper, more mystical meaning than what appeared in the text, he was unable to comprehend it fully.
• The investigators' first copy of the book was lent to Talbot Estus, who burned it on the night of his suicide. A replacement copy was later found in Lawrence Bacon's home, an then further copies were later found during their investigations.