There are 20 Field Manuals in the War Story "Through Mud and Blood" of Battlefield 1. Finding all collectible locations unlocks the "Enough for a library" trophy / achievement.
Each Field Manual is saved instantly when you pick it up - you can quit out to the main menu immediately without losing any progress. You don't have to pick them up again when you die and none of them are missable.
In mission select you can check how many Field Manuals you have found in each mission and how
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