A writer in the Quarterly Review for January 1874 says:

No literary event since the war has excited anything like such a
sensation in Paris as the publication of the Lettres a une Inconnue.
Even politics became a secondary consideration for the hour, and
academicians or deputies of opposite parties might be seen eagerly
accosting each other in the Chamber or the street to inquire who this
fascinating and perplexing 'unknown' could be. The statement in the
Revue des Deux Mondes that she was an Englishwoman, moving in
brilliant society, was not supported by evidence; and M. Blanchard,
the painter, from whom the publisher received the manuscripts, died
most provokingly at the very commencement of the inquiry, and made no
sign. Some intimate friends of Merimee, rendered incredulous by
wounded self-love at not having been admitted to his confidence,
insisted that there was no secret to tell; their hypothesis being that
the Inconnue was a myth, and the letters a romance, with which some
petty details of actual life had been interwoven to keep up the
mystification.

But an artist like Merimee would not have left his work in so unformed a
state, so defaced by repetitions, or with such a want of proportion
between the parts. The Inconnue was undoubtedly a real person, and her
letters in answer to those of Merimee have just been published by Messrs.
Macmillan under the title of An Author's Love.

Her letters? Well, they are such letters as she might have written. 'By
the tideless sea at Cannes on a summer day,' says their anonymous author,
'I had fallen asleep, and the plashing of the waves upon the shore had
doubtless made me dream. When I awoke the yellow paper-covered volumes
of Prosper Merimee's Lettres a une Inconnue lay beside me; I had been
reading the book before I fell asleep, but the answers--had they ever
been written, or had I only dreamed?' The invention of the love-letters
of a curious and unknown personality, the heroine of one of the great
literary flirtations of our age, was a clever idea, and certainly the
author has carried out his scheme with wonderful success; with such
success indeed that it is said that one of our statesmen, whose name
occurs more than once in the volume, was for a moment completely taken in
by what is really a jeu-d'esprit, the first serious joke perpetrated by
Messrs. Macmillan in their publishing capacity. Perhaps it is too much
to call it a joke. It is a fine, delicate piece of fiction, an
imaginative attempt to complete a real romance. As we had the letters of
the academic Romeo, it was obviously right that we should pretend we had
the answers of the clever and somewhat mondaine Juliet. Or is it Juliet
herself, in her little Paris boudoir, looking over these two volumes with
a sad, cynical smile? Well, to be put into fiction is always a tribute
to one's reality.

As for extracts from these fascinating forgeries, the letters should be
read in conjunction with those of Merimee himself. It is difficult to
judge of them by samples. We find the Inconnue first in London, probably
in 1840.