Percy was a playwright and a poet, and the first letter concerns some congratulations on one of his works, a play called "The Scarecrow." Belle writes to him in 1911 to make sure that he has read a review of his play in that morning's paper. She says it is "one of the best criticisms of a play" that she has ever seen. There's not much more to it; the letter is complimentary and maybe a little schmoozey. The reason we're looking at it isn't because it's particularly interesting, but because Belle herself is fascinating.
Belle was the personal librarian to the financier J.P. Morgan, which means that she built his rather famous library of rare books and manuscripts. She spent an enormous amount of Morgan's money and moved in some very intense social circles. By the time she wrote to Percy, she was already a prominent socialite and had earned a reputation for being clever, charming, fashionable, and an absolute force of nature in the auction house. A few months after this penning this letter, she would make waves by purchasing a ridiculously rare printing of Le Morte d'Artur for $41,800. Needless to say, she held her own in a profession dominated by older white men.
Belle was born to African-American parents but, following their separation, she, her mother, and her siblings would begin to pass as white, a move that was both risky and advantageous for them. Certainly it was not a fact that came to light during her lifetime. Belle actually burnt all of her personal papers before her death, so professional correspondence and the letters she wrote to others, like this one to Percy MacKaye, are all that remain.
There are more letters from Belle to Percy here--dated almost fourteen years later. At this point, Belle was the director of the Pierpont Morgan Library, which she had helped to turn into a public institution. This time, she's responding to an inquiry from Percy, who has asked her about some autograph letters in his possession that he has high hopes for. She has to send him her condolences. They simply aren't worth very much.
To dig through the MacKaye Family papers, inquire about ML-5. For Belle's letter specifically, ask for Box 39, Folder 9.