Record Changes in Height of Book Stack - Best Sensor?

Using Arduino Project Guidance
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I offer 3 plans;-

Plan A.

Digital Calliper and Arduino, measure Height of Book Stack directly. modify Calliper as book holder

Plan B.

Ultrasonic distance sensor and Arduino, measure Height of Book Stack indirectly by leftover Height.

Plan C.

RFID and Arduino, get which books in the shelf, u could get height.