Earlier this year the Berlin State Library acquired a Felix Mendelssohn autograph from a private collection. The pages (4 pages, 8 sides) contain sketches of Mendelssohn’s Piano Concerto No. 1 and his composition Die erste Walpurgisnacht.
The acquisition is significant because researchers had not previously had access to this material, which sheds light on the early stage of the composition process of these works.
You can see a sample of the Mendelssohn manuscript at the Berlin State Library website (click on image for a larger view).
Here you can a full performance of Die erste Walpurgisnacht.
Berlin State Library press release on Mendelssohn acquisition at http://staatsbibliothek-berlin.de/aktuelles/presse-news/detail/article/2016-01-26-2137/
Program notes on Piano Concerto No. 1 http://redwoodsymphony.org/history/prognotes.aspx?ID=177
Barenreiter’s description of its annotated score of Die erste Walpurgisnacht at http://www.takte-online.de/en/orchestra/detail/browse/11/artikel/mendelssohns-die-erste-walpurgisnacht-in-kritischer-neuedition/index.htm?tx_ttnews%5BbackPid%5D=549&cHash=0fd5be150a4847b112725f7dd9bf84f4
Image attribution: Lithograph of Felix Mendelssohn-Bartholdy https://commons.wikimedia.org/wiki/File:Felix_Mendelssohn-Bartholdy_(AMZ_1837).png