I haven't stumbled across one in a very long time.
I think internet content -- VSLs, blogs, etc. -- have replaced the long-form sales letter.
...And (dare I say it?) -- tl&dr is a very real thing in the internet age. Yes, if you can make a sales letter interesting enough, some will read it. But in the age of "tl&dr", that's becoming harder and harder to do.
Other than pieces like brochures (which your prospect actually requests), sales writing is becoming "slicker" and more "soft sell" -- which I think signals the downfall of the classic long-form sales letter.
I think one of the ramifications of this is new copywriters should update their "go to" examples of great sales writing.
Sales writing, like everything else in marketing, is changing, thanks to the internet. Sales principles may not change much, but sales writing is changing drastically. What do you think?