Regis de Trobriand was a regimental commander who rose in responsibility. Post-war he was thought highly enough by Grant to be offered a colonelcy of the 31st Infantry. He served in the Dakotas and there are three books, two of which were by him and the third composed of his letters. If you check the index of my book, you will find some interesting observations by Trobriand on sharpshooting.
More info on Philippe Régis Denis de Keredern de Trobriand (For some reason this is often shortened to Regis De Trobriand. Can't think why).
Born into the French aristocracy (He was the son of Joseph de Keredern de Trobriand, a baron who had been a general in
Napoleon's army(!), he the only Civil War general to have that honor) he settled in New York as a novelist, newspaper owner, editor and translator (though his training back in France had been as a lawyer). He was a prominent member of society and as such at age 45 shortly after the war broke out (and after he was required to take US citizenship) he was seen as the right sort of man to be made Colonel of the predominently French immigrant 55th New York Volunteer Infantry. [The Lafeyette Guards.]
His first Battle was Williamsburg (in the Peninsula Campaign) but then he came down sick and was absent for months. He was unlucky enough to return in time for Fredericksburg, Chancellorsville and of course Gettysburg where he valiantly and competently commanded a brigade in an eventually doomed attempt to hold the Rose Woods against repeated overwhelming Confederate assaults.
Commissioned Brigadier General of Volunteers in '64 he varied between brigade and divisional command during the rest of the war. (Obviously this letter was during one of the times he was a brigade commander).
After the war he returned to France only to learn that U.S.Grant had appointed him Colonel in the Regular Army (31st Infantry). This is really quite an honor considering who he is preferred to! He also turns round and promptly asks for a sabbatical to write another book. This shows how highly he is regarded in that it is immediately agreed to! It is especially unusual because he didnt go to Hudson High (West Point) and there was much prejudice against such types. He does (eventually) join his new regiment and briefly takes part in the Indian Wars and the military occupation of the South.
In 1867 he is Commissioned Major General of Volunteers though by then that is a mostly honorary sort of thing. Again it suggests he is held in high regard.
After his service he settled in New Orleans, wrote another book and died in 1897. He is however buried in Sayville, New York.
And as an admirer of his I would gladly pay money for that signature. Not a lot but some for sure. I would however advise you not to sell it (and am guessing you arent intending to) for in my opinion it increases the value of the rifle and the two should not be separated. Provenance and all.