IMDb: Marie Belloc Lowndes: Filmography. Several films have been based on her books and stories.
Popeau Intervenes: A detective story from 1926. Short story.
Spartacus Educational: Marie Belloc Lowndes: Biography of the novelist and suffragist.
The End of Her Honeymoon: Mystery. In plain text, or as a zip file, at Project Gutenberg.
The Lodger: Novel. In HTML, at World Wide School. Each chapter has its own file, linked to previous, next, or any other chapter. If you are using a graphical browser, this is the online equivalent of a "large print" edition.
The Lodger: Novel based on the story of Jack the Ripper. Available in plain text, or as a zip file, at Project Gutenberg.
The lodger (1912): Short story version. At Gaslight.