What's the literary term for an instruction to the reader at the beginning or end of a book?

Books and other texts can have a motto at the beginning (or at the end), or a dedication. But they sometimes include a command, directive or instruction to the reader as prologue or epilogue. It could simply be “Go and spread the word” or “Every time you see X, think about Y” or “Keep Z in your prayers”, or something quite a bit longer.

Is there a specific word for this?

Solution 1:

An envoi.

Definition by Wikipedia: An envoi or envoy is a short stanza at the end of a poem used either to address an imagined or actual person or to comment on the preceding body of the poem.

Solution 2:

Epigraph:

a quotation set at the beginning of a literary work or one of its divisions to suggest its theme

The word's Wikipedia entry provides the following definition (as well as illustrative examples):

In literature, an epigraph is a phrase, quotation, or poem that is set at the beginning of a document or component. The epigraph may serve as a preface, as a summary, as a counter-example, or to link the work to a wider literary canon, either to invite comparison or to enlist a conventional context.

Epigraphs are flexible and I expect that pithy phrases including commands, directives, and instructions will qualify.