Are there first person accounts of monothematic delusions? Monothematic delusions are delusions that concern a single topic (Davies et al. 2001). They are often the result of acquired brain damage and include the Delusional Misidentification Syndromes, such as Capgras delusion (which involves the conviction that a close relative or spouse has been replaced by an impostor), as well as Cotard delusion (which may involve the conviction that one is dead) and somatoparaphrenia (which involves the conviction that a limb or an entire side of one's body is not one's own).

While there are untold numbers of first person (experiential) accounts of schizophrenia and, therefore, schizophrenic delusions (which usually present as polythematic), from Daniel Paul Schreber's Memoirs of My Nervous Illness (1903) to Elyn Saks's The Center Cannot Hold (2007) and various accounts published in Schizophrenia Bulletin, I have not come across first person accounts of any monothematic delusions so far.
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).