Of late, I am frequently noticing these particular books on challenges. For some works, the reason is obvious; they DO appear often on challenges. But there are other mechanisms that seem to be in play. I finally read a particular book and then a more obscure book rises up to replace it in my consciousness, demanding to be read.
The Threepenny Opera --- Threepenny Novel
Dracula --- Dracula's Guest
Black Beauty --- The Black Stallion (black horses on the brain syndrome)
Or I complete a book or two of an author's output, only to have other unread works by the same author spring up to demand attention. In the present challenge this means the works by O'Connor, Vonnegut, Hemingway, Marlowe, Winterson, Bernhard, Steinbeck, Saunders, Vuong, Greene and others. In some cases you might even be able to guess which specific book(s) I have recently completed.