Amazon tells me that the book was Overcoming Chronic Fatigue by Mary Burgess, and it was recommended to me as the best book for CFS by a close friend who's a consultant psychiatrist.

As I remember, the first half of it describes what was happening to me so well that I could have written it myself, and the second half (the treatment) seemed weirdly irrelevant. I gave it to Oxfam in the end (the usual fate of my books) so I can't check what she actually said about neurasthenia.

The ngrams technique is brilliant! Actually Fibromyalgia seems to have started in '66. I am very interested in the fact that both things have fallen since 2000, any idea what that would mean?

Also, could you try some of the other things I'm claiming? I have done, and found what I expected, but if I say what to search for, that will look like I'm leading you.

I am certain to have been imprecise about other issues. I've only been thinking about this for three months, and I was writing what I thought, and now I can't distinguish my own writing from my memories or the truth! And in my original narrative I glossed over any details that detracted from the story, while being careful not to say anything I knew to be untrue or miss out anything I thought relevant. If I have been deliberately imprecise in misleading ways you have my word that it was unconscious bias, for what that's worth.

All changes will have happened slowly, and new 'syndromes' take a while to be accepted and given names, so the raw ngrams graph you link to looks as I'd expect up until 2000. The fall after, I can't explain, and the existence of Fibromyalgia in 1966 is a surprise too.

Looked up Billewicz' paper and it was published in 1969.