I'd agree RB did put some effort into the letter.... by copying various passages from books. The passage that starts with... "Frederick was perturbed when admitting to himself how little he knew of feminine psychology. An intelligent woman like his wife would have talked nineteen to the dozen, noted voice inflection, eyes and mouth expression and, having added"... was taken directly from a book named Venom House" by Arthur W. Upfield first published in 1953. Except Frederick was Bony.
The Answerth family's mansion seems to deserve its nickname of Venom House - perhaps because of its forbidding setting, an island in the centre of a man-made lake, its treacherous waters studded by the skeletons of long-dead trees. Perhaps it's because of the unquiet ghosts of the Aboriginals...
books.google.com.au
In another passage with "it doesn't pay to ride a theory too hard, you lose sight of the reality" is taken from a well known book Rich Man Poor Man" by Irwin Shaw.
I'd guess there are quite a few other snippets that he stole from legitimate authors.
IMO