This quest item is necessary to make any version of Raven's armor and is integral to the Armour quest, irrespective of the path chosen by Geralt.
These notes can be found in a sarcophagus in the striga's crypt on the island in the swamp cemetery.
Notes of an Elven minstrel
The style of the text is beautiful. It is said that a talented minstrel's song can imbue a suit of armor with magic.