In normal English usage the word “impromptu” means “without being planned” or “without being rehearsed.” However, etymologically, it comes from the Latin in promptu, which means “in readiness,” implying that the performer of the act is “ready,” even if the act itself is not preconceived.

Taking the whole sentence literally, it gives the impression that Gurdjieff, delivering the final draft of the book for publication, suddenly thought: “Hmm, maybe I should add a little advice at the beginning on how to read the book.” Then, concluding it is a good idea, he immediately dashes off a few paragraphs. The sentence in square parentheses is then added by someone else by way of explanation.