Literary Crossovers

Rider Haggard's She And Allan is equally a volume both of its author's She Trilogy and of his Allan Quatermain series just as Edgar Rice Burroughs' Tarzan At The Earth's Core is equally a volume both of its author's Tarzan series and of his Earth's Core series. Are there any other examples of precisely this kind of literary crossover?

Poul Anderson did not interconnect any two of his series in this way. There is no Van Rijn And The Time Patrol. Van Rijn does cameo in the Old Phoenix story, "House Rule," but his role in that story is too small to make it count as equally an installment of the Technic History or to cause the story to be included in a van Rijn collection. Anderson's time travel novel, There Will Be Time, incorporates the period covered by his Maurai future history and they even have a character in common but this is not two independent series intersecting.

Of course, Anderson's works are not merely self-referential. They also include an untold tale of Dr. John Watson and other Literary References.