Collect all 50 and they form a murder confession. Franklin also unlocks the Strangers and Freaks mission "A Starlet in Vinewood".
Letter Scraps can be collected after the story mission "Repossession". They are not shown on the game map.
There are 50. 100% completion needs every one.
One is on top of an LSIA tower. You need a helicopter or a parachute.
Near the Altruist Cult camp, the cultists are hostile to outsiders. Do not cut through the camp. The ridge or the hiking trail is the safer approach.