"read the first few pages, and it's actually really interesting because I understand the examples. Those stories are then applied to Psychology principles, which makes the principles so much more clear." (I need this for my psych class)
Also, Lily called Sirius 'Padfoot' in the letter, the name only his closest, dearest friends knew. The Marauders were such a tightly knit family, Sirius would certainly have mourned deeply enough to carry the letter even through Azkaban. Gonna go sob now.