This collection consists of 273 letters, most of which are from Marquis Astolphe de Custine (1790-1857) to his close friend, María de las Mercedes Santa Cruz y Montalvo, comtesse de Merlin (1788-1852). Several letters dated after Comtesse Merlin's death are addressed to her son, Comte François Xavier Pierre Christophe Dieudonné Merlin (1814-1900). Cuba's first female published author, a socialite, and singer, María de las Mercedes Santa Cruz y Montalvo was born into an aristocratic family in Havana, though she lived mostly in Madrid. In 1811, she married General Count Christophe Antoine Merlin, one of the commanders of the Imperial troops during the Spanish campaign. While living in Paris, in the Rue de Bondy, Comtesse Merlin held one of the most popular music salons in the capital. Custine was one of her many frequent guests.

The analytical and insightful observations found in Astolphe de Custine's travel writing are also found in his correspondence, the bulk of which date from 1848, 1849, and 1854. Reflecting on a myriad of topics, Custine's letters to Merlin offer his insight as a member of the aristocracy into the political, intellectual, social, and cultural happenings in Europe during a time of great change; they also provide a good deal of insight into his own character.

A passionate traveler, many of the letters were sent from various locations, including France, Switzerland, Italy, London, where he attended the first World Exposition, as well as from Saint-Gratien, his estate outside Paris. A sizeable portion of the letters were sent from Italy as Custine travelled from Rome to Naples, writing in great detail about the revolution of 1848 and events that took place at the beginning of the Risorgimento.

Along with expounding on political life and structures in France, Europe, and Russia (note, no letters date from 1839), much of Custine's correspondence concerns his passion for art, music, including the rise of Italian opera, literature, the intellectual and scientific debates of the day, and his encounters and relationships with many associated figures. In the music realm, these include composers such as Gaetano Donizetti, Giuseppe Donizetti, Giuseppe Verdi, Hector Berlioz, Frédéric Chopin, and Felix Mendelssohn, as well as some of the most well-known singers of the time.

A devout Catholic, Custine also documents the relationship he fostered with Pope Pius IX, whom he met several times before, during, after the pope was exiled to Gaete. Other topics found in the letters include his interest in homeopathy, his writing endeavors, and his thoughts regarding the French moralist tradition.