Oh, that letter is great! I feel like she must have enjoyed writing the ad-agency parts more than she suggests, because they're so detailed (in characterization as well) and so true to life.
I could read a whole series set in the agency, with all its petty drama and delightful characters.
Me too! Imagine a whole series starring Miss Meteyard and Mr. Ingleby. "Courses of the world, please, not curses." And Peter's wholehearted irritation about having to put away his brilliant advertising idea for a moment and go back to detecting, and the office boys' back and forth, and the amazing tension of the cricket match.

and seeing DLS do the same thing makes me feel better.
Yes indeed, oh dear. (Also, imagine DLS in fandom...).