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When there are many objects interacting, such as three or four books stacked on each other, many people, when drawing the free body diagram (FBD) for the bottom book, want to draw forces from all of the other books. However, if we draw our system boundary we see that the only book crossing the boundary of the bottom book is the one right above it. The other books do not apply a force dirrectly to the bottom book so we can't include any forces from them on the bottom book.
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One thing people forget when doing a free body diagram is that the choice of axis is not set in stone. This means that every time we draw a FBD we must include the axis that we are using to solve our problem.
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Only forces go on a FBD, do not draw velocity or acceleration on a FBD.
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Each “separate” system, as we define them, should have a seperate FBD that only include forces that act on that specific system. That specific system could contain multiple objects.