CROSSOVER MADNESS

I've read none of the works of L. Frank Baum except the first OZ book, but on researching the author I learned that, thanks to the almost immediate success of that series, he established some loose linkages to other attempted franchises. For instance, a couple of characters from the 1909 ROAD TO OZ show up in the 1912 SKY ISLAND, the second of two books which the author had to drop because they didn't sell well enough to continue. He later had two characters from that short lived series in the 1915 SCARECROW OF OZ, and Wikipedia lists a number of projects which aren't technically Oz books but have loose connections to that universe. So Baum stands as one of the first crossover-makers of the twentieth century.