I'm not talking about currently popular things like Harry Potter or His Dark Materials or A Series of Unfortunate Events--things that people come across in mainstream culture--though some or all of them may well join the ranks of the stumbled-upon someday--or about classics that are acknowledged in the outside world, like LotR, Foundation, Dune, or The Chronicles of Narnia. The book that started me thinking about this was Robin McKinley's Beauty--nearly unknown outside of the genre, but within the genre, at some point as you get into the fandom, someone somewhere is going to say, "Oh, have you read Beauty?" (For the record, I followed the advice and picked it up, and really disliked it.(EDIT: To my embarassment, I realized the Beauty I so disliked was Tepper's, not McKinley's. It's been a while since I read them!)) To some extent, the Ender books are like this, though the release of current entries in a series puts it in a slightly different light, closer to the popular stuff.
So, question up for discussion: Which SF/F books do readers in the genres think of as perennials, the books that every speculative fiction reader comes across sooner or later?