Beethoven died March 26, 1827. As his effects were sorted, a compartment of his desk fell open, revealing a 10-page love letter. Beethoven had poured out his heart with passionate love for an unnamed woman. Among his several loving names, he calls her his "Unsterbliche Geliebte" (Immortal Beloved). The letter is dated July 6-7. Later analysis determined that the letter was written in 1812 at Teplitz (now Teplice).
Did Beethoven ever send the letter? Apparently not. There was no envelope with the letter, which seemingly would've been the case if the woman had returned the letter.
Who was she? Beethoven never married but had affections for several women. Among them, Antonie Brentano and Josephine Brunswick (pictured) are strong contenders, based on similarities of wording with known love letters. Josephine's sister Therese is another. Beethoven dedicated the Moonlight Sonata to Julie Guicciardi, who is another possibility. I have a book that builds the case for Dorothea von Ertmann, a talented pianist who performed Beethoven's works.
The letter is one of the most famous aspects of the composer's life. A 1994 movie about the letter starred Gary Oldman. The woman is also featured in the Trans-Siberian Orchestra's rock opera "Beethoven's Last Night."