There are 17 Marvin Zucker's Battle Journals in Dying Light. They are one of text collectibles and are required for the "It's All in the Writing" trophy or achievement (34 Notes, 17 Battle Journals, 16 Voice Mails). The flags and statues are not required.
Before attempting to collect these Journals you should complete the story! Some of them are locked away in areas that require certain story progress. You can still access all areas after beating the game by using fast travel (nothing is missable).
To see how many you have found, open your inventory and press Square (PS4) / X (Xbox One). There you can see an overview of all the text collectibles you have picked up. They all have their own number, so that you can track what you are missing.