I would suggest a pair of low frequency capacitance sensor. The plastic 
has a dielectric constant of at least 2 and more likely 4 or higher and 
should be easy to detect. If the pages are thick enough the capacitance 
will vary in large steps and be easy to detect differentially (assuming 
the sensors are either in the covers or at the 'core' - where they are 
less likely to detect someone's hand or finger).

Peter
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