There are total of 10 Letters in the whole Game. You can not miss a Letter, via Chaptermenu you can any Time collect your missing Letter. The Letters are immediately saved, you dont need finish the Chapter or reach a Checkpoint.
In Chapter 8 you have to complete a Sidemission to get the Letter. Take the Sidemission and bring the Medicine back to the mission givers.
Here are the 10 Letter Timelinks with my all in one Videoguide for all 172 Collectibles.02:31
- Oskars Letter (Chapter 1 - Deathshead’s Compound)07:39
- Izabelas Letter (Chapter 2 - Asylum)09:18
- Romans Letter (Chapter 3 - A New World)14:40
- Erdmanns Letter (Chapter 4 - Eisenwald Prison)26:18
- Bobbys Letter (Chapter 7 - A Mystery)27:43
- Sidemission start for the Letter (Chapter 8 - Camp Belica)29:00
- Medicine for the Letter Sidemission (Chapter 8 - Camp Belica)30:30
- Christas Letter (Chapter 8 - Camp Belica)33:33
- Charlottes Letter (Chapter 9 - New Tactics)36:15
- Ingrids Letter (Chapter 10 - berlin Catacombs)54:39
- Dieters Letter (Chapter 13 - Lunar base)59:53
- Maxs Letter (Chapter 15 - Under Attack)