Documentation for ed is available on the Internet, but I found it a great help to take Richard Gauthier's USING THE UNIX SYSTEM (1981) with me when I reported for jury duty in Portland, Oregon. His 30-page discussion of "the editor" is thorough and gave me some sense of the power of this pioneer text editor (cut & pastes, for example).
As I said, what drives my mule-like early morning routine is word count. The text editor ed has no internal word count tool (through dropping back to the command line gives, of course, wc). What I had to do was quite simple: I converted byte-counts (which ed does with each write to the file) into word equivalents. So if my style of writing runs 5.6 characters per word, then a word goal of 1,000 words is simply 5,600 bytes. Every day, I set my target byte count and once there, I quit.
In less than three months, I finished a 72,000-word novel draft and give ed credit for not slowing me down. Based on my experience writing novels with plain text editors (vim, geany, and now ed), I understand how few computing resources are needed to take manuscript composition off a typewriter and put it on a personal computer. The advantages of the latter are several, including less retyping, easier revision, and portability among different systems. Whether going from typewriter to personal computer makes for better writing I'll leave to others for comment.
What doesn't make for better writing is confusing text on demand (that daily word count that grows to a manuscript) with desktop publishing. Desktop publishing makes so many word processors into distracting choice-laden software tools. Obviously, there is a place for a manuscript as PDF file compliant with appropriate Acrobat Distiller settings, but that ends, not begins, the process. I like to think I'm not putting the cart before the horse.
So why would I recommend ed for a wordsmith? I'd say it comes down to just enough computing resources to do the job. WYSIWYG word processors have a cost and intuitively I think there's cerebral bus contention between flow of words onto the screen and keeping a handle on where the mouse arrow is (among other things).
But then perhaps I've a "less is more" bias (I have a car with nonpower steering — better road feel; I ride a fixed single-speed bike — ditto). That feeling is the sum of things there (and things left out). When I ride my fixie bike, it seems to know why I ride. Similarly, when I invoke ed, the text editor, it seems to know why I write. An illusion, sure, but also a harmony that goes with being responsible for all of it and staying focussed (without any distracting help balloons!).
One of Charlie Dickinson's novels is available for download at cetus-editons.com.