I never got around to actually learning Morse Code, but I think the English language corpus is probably dramatically different from the messages that are actually sent via Morse Code. My understanding is that abbreviations are commonly used, greatly skewing the letter frequencies: https://en.m.wikipedia.org/wiki/Morse_code_abbreviations
There are also a large number of Q codes that cover practically everything related to communication and also naval, aeronautic and rescue concerns. Today much of this isn't in use, but amateur radio operators use a subset of these routinely; a typical fast Morse code exchange is often composed almost entirely of Q codes and call signs.
I became aware of Q codes through their use in the book Seveneves. It provided a brief glimpse into a subculture that I've never had much contact with, and I ended up in a bit of a Wikipedia crawl.
I was lucky enough as a kid to inherit my grandfather's collection of old sci-fi books. I first became aware of Q codes through the short story "QRM Interplanetary" from 1942 (https://en.wikipedia.org/wiki/Venus_Equilateral).
I find it interesting that Q codes have changed so little over the last century. I have some ham radio friends who will use Q codes when talking face-to-face with each other, and the codes they use are basically the same ones from the original list.
I'm glad you commented. I was sitting here racking my brain, trying to remember what book I'd read that involved Q codes. Then I saw your comment, and it was indeed Seveneves.
Military morse users would predominantly send ciphertext, so the letter distribution would (hopefully) be random.
Professional morse users often sent very condensed messages using a variety of abbreviated formats. For example, here's an excerpt from an NOAA aviation weather forecast:
WA CASCDS WWD
CSTL SXNS...SCT025 SCT050. 06Z SCT020 BKN050 TOP 100. OTLK...MVFR
CIG.
OLYMPICS...BKN-SCT060 TOP 080. ISOL -SHSN. 04Z SKC. 08Z BKN050
OVC150 TOP FL200. OCNL VIS 3-5SM IN SCT -SHSN. OTLK...IFR CIG
SHSN.
Wired Love: A Romance of Dots and Dashes (1879) describes a long distance romance conducted "online", from the point of view of a female telegraph operator, where neither party knew what the other looked like or even the sound of their voice. Describes the virtual communities of telegraph operators and the culture surrounding them. It's a wonderful book.
Copies can be found at archive.org, Gutenberg, and Amazon.
What's also interesting to me are the telegraph hacks during the American Civil War. Tapping into enemy wires and sending misleading orders, or just listening in. Here's some discussion on it:
