The slip of paper contains a publisher's note from J.P. Lippincott Company of Philadelphia. It states: "This New Story is the longest and most important work by MISS CORELLI published since the 'Sorrows of Satan.'"
Corelli (1855-1924) was a popular author who, according to Wikipedia, "sold more copies than the combined sales of popular contemporaries, including Arthur Conan Doyle, H. G. Wells, and Rudyard Kipling." Her Faustian horror novel The Sorrows of Satan (1895) was "widely regarded as one of the world's first bestsellers."
She wrote more than two dozen novels, including A Romance of Two Worlds, The Soul of Lilith, The Mighty Atom, The Strange Visitation of Josiah McNasson: A Ghost Story, and The Secret Power. As you might guess from some of those titles, some of her books fell within the genre of what was then termed "scientific romance" and would today be called "science fiction."
Given how well her books, including The Sorrows of Satan, sold, I suppose it makes sense that Lippincott would include this enticing green slip inside Boy, which, by itself, doesn't exactly have a title that cries "bestseller." I bet they wish she had titled it Beelzebub's Boy.