In Hugh Kingsmill's "Frank Harris" (1932) he describes how in the 1890s, in the Saturday Review, Harris wrote a series of attacks on the German regime and Wilhelm in particular, and then goes on to say that Wilhelm mentioned these "in his book on the causes of the war". I have been unable to identify what book Kingsmill meant, and thus locate the quote about Harris. (Part of the problem being, I think, that the automated scanning software used by online book repositories generally cannot cope with old German blackletter type).
I checked Wilhelm's memoirs and can find no mention of either Harris or the Saturday Review. I have performed searches on Google Books and the Internet Archive, all to no avail. I wonder if the book in question may have been written by someone else than Wilhelm, but I don't know the possible sources at all well, and my German is minimal, so I am now stuck.