'saepe stilum verte, bonum libellum scripturus.'
Benissumus's answer here is 'Use the eraser often, you are about to write a good little book.' Which sounds right. But isn't there a case (pardon the pun) for scripturus being in the vocative? i.e. scripture?
My reasoning is that verte is singulare imperative, so you're talking to the person who is about to write, and when you are talking to someone, you put their name (if present), and any adjectives in agreement into the vocative.
'You, who are about to write a good little book, use the eraser often'
Am I missing something?