Just a heads up y'all... I think there may be a minor bug with the documents / letters / photos.
PROBLEM
Whenever you read a letter (or view a photo) that you have acquired... once you select
"Put Away"... sometimes a new location / side-mission will be unlocked on your map. But it doesn't always work.
For example: in a saloon in Valentine you will meet a writer named Calloway, who asks you to track down a group of 4 famous Gunslingers. He gives you a photo of each one. Basically, he wants you to find them & kill them... and then take a picture of each of them.
Important: Most of these Gunslingers have a
'unique weapon' they will drop, when killed. You want to pick these up.
Whenever you get a new photo/book/or letter, you might open the item right away (or wait awhile

) >> then read it / or view it >> then just put it away, and completely forget about it. But viewing the 4 gunslinger photos & just putting them away doesn't always unlock their related side-missions, for some strange (buggy) reason.
I viewed these photos when I first got them... even flipped them over... then selected 'Put Away'.... but nothing happened.

However, when I went back and opened my inventory & looked at the photos a 2nd time (a few days later), I noticed something...
SOLUTION
If you use the
D-Pad to scroll through your 'document' inventory, it should unlock (previously unknown) side-missions that were supposed to work earlier. Including this one & others.
STEP 1 - Open your satchel
STEP 2 - Select the 'Documents' menu tab.
STEP 3 - Open an item (like the Gunslinger photos, for example)
STEP 4 - Use the D-Pad to open the next item.
Use the D-Pad method (whenever possible) to open ALL the items in your 'Documents' tab.
This will probably unlock any (related) side-missions that were previously unavailable. (i.e. unless a particular side-mission is not supposed to be available until your reach a certain point in the 'main story' missions)
Hope this helps.