Are there first person accounts of monothematic delusions?
Since asking the question, I was able to locate a first-person account of monothematic delusion, namely, of denial of ownership of one's own limbs (somatoparaphrenia/asomatognosia). It is due to the neurologist and writer Oliver Sacks, who in his fourth book A Leg to Stand On (1984) described his recovery after a fall in a remote region of Norway in which he injured his leg. Following surgery to reattach his quadriceps muscle, he experienced an emotional period in which his leg no longer felt a part of his body. He describes his confusion, seeing the “disowned” plastered limb (Sacks 1984: 45,49, 49–50):


  I had felt the leg in front of me . . . but now I could see it wasn’t
  there at all but had got shifted and rotated. . . . I had a
  sudden sense of mismatch, of profound incongruity –
  between what I imagined I felt and what I actually saw,
  between what I had “thought” and what I now found. I felt, for
  a dizzying, vertiginous moment, that I have been profoundly
  deceived. . . .


The experience of touching the leg was “inconceivably shocking and uncanny”:


  I seemed to have lost “my leg” – which was absurd, for there it
  was, inside the case, safe and sound – a “fact.” How could
  there be any doubt in the matter? And yet there was. On this very matter of “having” or “possessing” a leg, I was
  profoundly doubtful, fundamentally unsure.


This alarming state of uncertainty later resolves itself into
what seems closer to a fully delusional state:


  [the leg] became a foreign, inconceivable thing, which I looked
  at, and touched, without any sense whatever of recognition or
  relation. It was only then that I gazed at it, and felt I don’t
  know you, you’re not part of me, and, further, I don’t know this
  “thing,” it’s not part of anything. I had lost my leg. Again and
  again I came back to these five words: words which expressed
  a central truth for me, however preposterous they might
  sound to anyone else. In some sense, then, I had lost my leg.
  It had vanished; it had gone; it had been cut off at the top. I
  was now an amputee.


EDIT: I have found a first-person account of Cotard delusion in this blog post, thanks to Vaughan Bell who found it through Keith Frankish (both authors working extensively with delusions).