This line (in the version I am reading) is in Act 1, Scene 5. Lady Macbeth tells Macbeth that the expression on his face is too troubled and/or sinister. She uses a simile to compare his face to a book. A simile is a figure of speech in which two different things are compared to establish a similarity between those two different things. Similes use "like" or "as" to illustrate this comparison.
Your face, my thane, is as a book where men
May read strange matters. To beguile the time,
Look like the time; bear welcome in your eye,
Your hand, your tongue; look like the innocent flower,
But be the serpent under't. (I.v.60-64)
At this point, Macbeth's expression and behavior communicate ambition. Lady Macbeth recognizes this will come across as suspicious, so she wants Macbeth to look, act, and speak with humility and appreciation when the king arrives. Therefore, to deceive (beguile) the king (and others), Macbeth will act like appropriately ("Look like the time;"); that is, Lady Macbeth wants Macbeth to hide his ambition and act like a loyal subject, grateful for the promotion.
In Act One, Scene 5, Macbeth returns to Inverness and tells his wife that Duncan will be visiting them tonight. Lady Macbeth is excited about the possibility of her husband becoming king after he kills Duncan. Lady Macbeth looks at her husband and says,
"Your face, my thane, is as a book where men may read strange matters" (Shakespeare 1.5.53-54).
As was mentioned in the previous post, the literary device that Shakespeare employs in this line is a simile. A simile is a figure of speech that compares two things using the words "like" or "as." Lady Macbeth compares her husband's face to that of a book that men can read. This line also contains another literary device known as an idiom. An idiom is essentially a common phrase. It is common to say that a person's recognizable facial expression can be "read like a book." Lady Macbeth does not want the other Lords to recognize Macbeth's true intentions and encourages him to appear normal and delighted to be in the presence of the king.