Harry Potter & 'The Passion Year' Memoirs

Starts at POA (AU)/partly Marauders Era. While exploring the Map's layout of Hogwarts, Harry finds a secret room in which a collection of memoirs written by James Potter were left. Reading it, Harry discovers that Sirius is innocent but has no idea how to prove it without exposing a dark secret of his father's past. S/J with eventual J/L & S/R. AU but based on canon.