More Fruits of Solitude
Death is, but crossing the world, as friends do the seas; they live in one another still. For they must needs be present, that love and live in that which is omnipresent. In this divine glass they see face to face; and their converse is free, as well as pure. This is the comfort of friends, that though they may be said to die, yet their friendship and society are, in the best sense, ever present, because Immortal.

William Penn, More Fruits of Solitude

I think this refers to the books because it saying that even though friends are dead, they are still with and in us. This can especially refer to the part in HP when the Resurrection Stone brings back Harry's friends and mentors. It also can be referring to how Dumbledore kind of reached out to Harry from his grave by leaving clues and hints.