"Clue" comes from "clew", a ball of thread. The modern sense of a guiding hint is linked to stories like Ariadne giving Theseus thread to escape the labyrinth.

Tsundoku is the practice of acquiring books and letting them pile up unread. The word carries no reproach — it combines 'piling things up' with 'reading', and Japanese usage treats the pile as fairly harmless.

It appears in print as early as the 1870s, originally as a mild joke about scholars whose collections outran their reading. The affection in it is the point: a stack of unread books is a record of intentions, curiosity and optimism, not a moral failure. The writer Umberto Eco kept a library of tens of thousands of volumes and argued the unread ones were the valuable part — a private reminder of how much there is still to know. English borrowed the word only recently, which suggests the need was there all along.