The speaker imagines making a book as a gift for "my old wife," and the whole poem is built from quiet promises: "I would make," "I would give," "I would undertake." Nothing is actually done yet. Everything stays in the hopeful, conditional mood of what might be.
The book is tied to "the cipher of our loss," a phrase that suggests something painful the couple shares but can't easily put into plain words. A cipher is a code, so the loss seems to need translating or careful handling. Against that heaviness, the speaker's offerings are small and humble: paper, a prayer for time, and the willingness to own "a small fact." There's tenderness in how modest these are.
The final choice, never to give the book a title, leaves it open so it can hold "anything one might write down." The mood is gentle, patient, and a little sad, but also generous. The speaker doesn't try to explain the loss. He makes room for it, and for whatever the two of them might still say.