Three short lines hold two quiet actions side by side: the sea changes color as evening arrives, and the speaker puts their books in order. The poem never says the two are connected, but placing them together invites us to feel that they are.
The title, "Collector," suggests the speaker is someone who gathers and keeps things. Sorting books is a small, careful, human task. The sea's shift at dusk is large and slow and out of anyone's hands. One is control; the other is something we can only watch. The speaker seems to find a gentle balance between them, tidying a little corner of the world while the bigger world changes anyway.
The mood is calm and contemplative, with a hint of wistfulness that comes with the end of the day. Nothing dramatic happens. The poem seems to say that paying attention, to the light outside and to the objects we love, is its own kind of quiet satisfaction.