There are 35 Personal Letters. in the (main) game, five in each mission (except mission 1 and 9). As always, some of the letters are very well hidden, and others are placed in easy spots. Fortunately, there are no more letters on the enemies, they are all lying around in the missions, on park benches, tables, chests and so on.
In the spoilers of each mission, you will find the map of the mission and the marked letters (blue dot / point), so they should be easy to find. If you have any problems or need help, videos for each mission are also available.
Maps:
I created these maps for myself to find all the collectibles and determine a good path / order for my videos but I think they are good enough to help you here as well. I might create new maps as soon as I have more time. (No guarantee). The letters are marked with a blue dot / point.
Mission 2 (Dead Drop)
Mission 3 (Sonderzüge Sabotage)
Mission 4 (Collision Course)
Mission 5 (Devil's Cauldron)
Mission 6 (Assault on Fort Rouge)
Mission 7 (Lock, Stock and Barrels)
Mission 8 (End of the Line)
Videos:
The videos are complete guides for the individual missions, so they show all the collectibles and not just the letters. In the spoiler of the missions you have the video, the timeline of the letters and an overview of the letters in the game, so you can find missed letters very quickly. The numbers behind the letters in the timeline match the sorting in the game.
Mission 2 (Dead Drop)*** Spoiler - click to reveal ***02:29 - 04-05 - Stone Eagle, Letter #1
03:50 - 07-10 - SMG Workbench, Letter #2, Stone Eagle, Document #2
05:21 - 12 - Letter #5
05:43 - 13-16 - Letter #3,
07:49 - 19-21 - Kill List, Letter #4, Document #4
Personal Letters:
#1 - A lost soul trembles
#2 - Missing you dearly
#3 - Surrounded by idiots
#4 - A withering tree
#5 - A nephew's concern
Mission 3 (Sonderzüge Sabotage)*** Spoiler - click to reveal ***01:01 - 03-05 - Hidden Item #3, #1, Letter #3
04:54 - 11-13 - Document #2, Letter #2, Document #1
07:31 - 14 - Letter #1
09:34 - 16 - Letter #5
11:26 - 20-21 - Letter #4, Document #5
Personal Letters:
#1 - Be Safe, My Dear
#2 - We Are Everywhere
#3 - Cross Of Lorraine
#4 - Missing Her Birthday
#5 - The Scars Of War
Mission 4 (Collision Course)*** Spoiler - click to reveal ***00:00 - 01-02 - Stone Eagle, Letter #1
01:08 - 04 - Letter #3
05:14 - 14 - Letter #4
06:45 - 18 - Letter #2
07:08 - 19 - Letter #5
Personal Letters:
#1 - A Gift, And A Name
#2 - I Will Miss You
#3 - What A Discovery!
#4 - So Boring!
#5 - I'm So Sorry
Mission 5 (Devil's Cauldron)*** Spoiler - click to reveal ***00:39 - 02-04 - Rifle Workbench, Hidden Item #3, Letter #1
02:38 - 05-06 - Letter #2, Document #3
04:00 - 08-09 - Stone Eagle, Letter #5
10:33 - 17-20 - Starting Location, Letter #4, Kill List, Letter #3
12:56 - 21-22 - Stone Eagle, Document #2
Personal Letters:
#1 - Location Exposed
#2 - A Childish Party!
#3 - I Miss You So Much
#4 - It Is All In Ruins
#5 - Be A Man, Not A Boy
Mission 6 (Assault on Fort Rouge)*** Spoiler - click to reveal ***02:25 - 06 - Letter #1
03:27 - 08 - Letter #5
05:50 - 11 - Letter #3
07:38 - 15 - Letter #2
11:54 - 19 - Letter #4
12:54 - 20-22 - Rifle Workbench, Document #5, Hidden Item #2
Personal Letters:
#1 - Just Do Your Job
#2 - I Am Fed Up
#3 - Something is Strange
#4 - Beatiful Views
#5 - Fools Everywhere
Mission 7 (Lock, Stock and Barrels)*** Spoiler - click to reveal ***00:00 - 01 - Letter #1
02:06 - 06-07 - Kill List, Letter #5
03:22 - 08 - Letter #2
05:44 - 13 - Letter #4, Optional Targets
06:47 - 14 - 2nd Sniper, Letter #3
Personal Letters:
#1 - What Do They Want?
#2 - Everyhting is Fine!
#3 - I Feel Uneasy
#4 - Lying is Necessary
#5 - Do Not Worry, Mother
Mission 8 (End of the Line)*** Spoiler - click to reveal ***02:55 - 07 - Letter #3
04:57 - 11-12 - Kill List, Letter #4
06:36 - 15 - Letter #2
08:28 - 17 - Letter #1
10:44 - 20-22 - Letter #5
Personal Letters:
#1 - Victory Is Imminent!
#2 - Home SOon, My Love
#3 - What We've Achieved
#4 - Burn After Reading
#5 - I Think This Is It
If you have any questions, just ask. And just in case you like to have everything in one video, here is a video that includes all collectibles. (added also below Target Führer DLC)
All in OneLights, Camera, Achtung - Target Führer DLC