Near the end of McCarthy's Blood Meridian, a dancing bear gets shot--it is but one more act of violence in a book full of senseless, meaningless violence. As I read the incident, I couldn't help but recall Hawthorne's "The May-Pole of Merry Mount," when Endicott orders a bear that was dancing with people to be shot.
Near the beginning of Shaw's Man and Superman, Ramsden tries to console Octavius over Whitefield's death by telling him "it's the common lot." Reading this, it is hard not to recall Gertrude's admonition to Hamlet that "Thou know'st 'tis common; all that live must die/ Passing through nature to eternity" (Hamlet dismisses her with "Ay, madam, it is common"; no matter how true it is, it is pretty shoddy consolation to tell a mourner that death is the common lot).
I don't know that either McCarthy or Shaw intended an allusion (though the parallel in the death of the dancing bear in Blood Meridian and "The May-Pole of Merry Mount" offers intriguing symbolic meaning). What's more, it doesn't matter if these parallels were intentional. As a reader, the connection has been made in my head. As I am reading, I can't ignore my memories of earlier works I've read: the parallels pop into my brain whether I seek them out or not. These parallels may mean much, or they may mean little and I might pass over them quickly.
But while reading one work, I have the memory of everything else I've ever read (or at least, everything that I remember of everything else I've ever read). As I'm fully engaged with a text, that text may remind me of other things I've read. It may provoke me to stop and consider the texts.
For reading, that means at least two things, I suppose. It means we rarely read in a vacuum: we bring ourselves and our memories of past reading to the reading experience. It also means that finishing a book does not truly mean finishing a book: that book could come back to you at a future moment, in unexpected context.