This secondary objective is given by Pawn 4 in act II.

Spoiler warning!
This article contains plot details that you may want to discover on your own.

Go down to the Barracks and open locker number 5 (Pawn 4's locker) to complete the objective. The book inside is a hint at who the traitor is.

Finding the book by itself isn't very helpful...
If you recycle the book, you'll get the Speak to Pawn 4 again first, or confront Pawn 32 directly secondary objective.
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