Rose Greenhow was a Confederate spy, who operated in Washington, D.C. in the first year of the Civil War. Her letter in cipher written on mourning paper is found on the Web (e.g., National Archives, World Digital Library). Since a brief search on Google did not turn up its plaintext, the present author deciphered it, only to find it had been done by others before (as expected).
Although the key to the cipher used by Rose Greenhow as of 1864 is found, e.g., here and here, difference in script style makes it difficult to apply it to the cipher letter of 1861. The following cipher table of actually used characters are reconstructed by the present author from another deciphered letter at The Rosenblog.
While the letter on mourning paper only uses symbols to represent 26 letters (plus one for "th"), the 1864 key includes more symbols for "ph", "sh", ..., "Lincoln", "infantry", "Pen Avenue", ..., and numerals.
The cipher letter on mourning paper turned out to be the message of 31 July 1861, shortly after the First Battle of Bull Run.
Ann Blackman's website has an image of another letter (beginning with "On 16th an order was given ...") from the same period here. Her book, Blackman (2005), also reproduces newspaper clippings with a message in cipher in the margin, part of which can be read "... numbers are marked by the engineer who built ... [y?]ou may rely on it .... nineteenth."
The Rosenblog has another cipher letter, beginning with "Fort 'Ellsworth,' situated about".
Fishel (2014) prints plaintext of Greenhow's nine letters in Appendix 4.
Rose Greenhow (1813 or 1814-1864) (Wikipedia) was a socialite in Washington, D.C. Fervent pro-Southern sympathizer, she agreed to become a spy for the Confederacy, when Thomas Jordan (Wikipedia), who managed formative Confederate intelligence service, was about to resign from the US army to join the Confederate army. Jordan gave her a simple cipher in April 1861 (OR, Series II, vol.2, p.564), the month in which the Civil War began at Fort Sumter, SC.
On 9 July and 16 July 1861, Greenhow sent messages to Confederate General Beauregard (Wikipedia), who won the First Battle of Bull Run on 21 July. Confederate President Jefferson Davis acknowledged contribution of Greenhow's intelligence to the victory. The above letter is written shortly after the battle and is concluded with "Don't give time for reorganizing."
On 23 August 1861, she was placed under house arrest (OR, Series II, vol.2, p.561, 566). Although she managed to burn the cipher, papers were seized. Among them were eight reports to Jordan, including the letter on mourning paper. Her house was put under surveillance by the Federal secret service. Although she managed to send a report to Jordan, the latter discredited it. He now knew that the cipher he had prepared for Greenhow, the first he had ever made, was not a good one and he was sure the Federal government had worked out the key. (OR Series II, vol.2, p.564).
In January 1862 Greenhow was moved to a prison and in May she was paroled and sent to Richmond (OR, Series II, vol.2, p.1351). Hailed by Southerners, she then took on a diplomatic mission to win support for the Confederacy and, running the blockade, went to Europe. She was well-received and her memoir published in London earned a huge amount of money.
In October 1864, when she was returning to the Confederacy carrying despatches, the British blockade runner she boarded was pursued by a US gunboat and ran aground. Fearing capture, she fled the ship by rowboat but was drowned. The above-mentioned cipher key as of 1864 was found on her body.
Ann Blackman (2005), Wild Rose (Google)
Edwin C. Fishel (2014), The Secret War for the Union: The Untold Story of Military Intelligence in the Civil War (Google).
The war of the rebellion: a compilation of the official records of the Union and Confederate armies ("OR") (Making of America, ehistory), esp. Series II, vol.2
The Official "Rebel Rose" O'Neale Web Site, esp. the five links under "Rose's Code" section
Rose O'Neale Greenhow (1863), My Imprisonment and the First Year of Abolition Rule at Washington (Documenting the American South, Internet Archive) ... Greenhow's memoir