Final answer:
Beethoven's Immortal Beloved is an unidentified woman who received a series of love letters from the composer. The mystery of her identity remains unresolved. Several sources, including Beethoven's personal letters, historical documents, and testimonies from his friends, have been used to try to solve this mystery.
Step-by-step explanation:
The Immortal Beloved refers to the mysterious woman to whom Ludwig van Beethoven wrote a series of passionate love letters. The identity of Beethoven's Immortal Beloved has remained a subject of speculation and debate among historians and music scholars. However, three sources that have been used to try to solve this mystery are Beethoven's personal letters, historical documents, and testimonies from his friends and acquaintances.