Books without a market sometimes find their way into what genteel bookmen call the "curiosa" shelf. Often this stuff is more or less specialized porn or "erotica." Often it's just too weird to fit into whatever system the buyer has; for example, the C.F. Russell booklets would find their way here unless the store knew an enormous amount about such things. And sometimes it will be both weird and what you could call porn, which is to say it exerted some kind of libidinal power over its owner at one point as a talismanic object, a beloved thing of more or less intimate fantasy, but isn't the normative naked lady sort of thing. Children's books often have the earmarks of this when you find them in either really good or really bad condition, but children's books are also a well-recognized market and so their fascination is pretty normalized. Same with comic books or 50s crime paperbacks or science fiction for that matter. A true curio's value is more obscure. Sometimes you can puzzle it out from provenance -- even a scissored-out carpet ad becomes a numinous relic if it was owned by Francis Bacon or, again, Joseph Cornell -- but often biographical cues are absent or dead ends. Yard sale fans are good at sniffing this stuff out before it falls back through the cracks of the world.
What are your curios? What will happen to them when you're gone?
In other news I think the work of Joel-Peter Witkin [pictured above: Costumed Inmate, Insane Asylum] still has things to teach me, although it isn't what I was getting out of it twenty years ago.