The proposal *probably* refers to [terahertz (THz)](https://en.wikipedia.org/wiki/Terahertz_radiation) imaging; THz is the band that is above microwave, but below infrared, between 100 um and 1 mm wavelength; often referred to as T-Rays.

Because of their relatively long wavelength the T-Ray penetrates well into non-conductive materials, and is far, far below the ionization threshold.  Thus it is safe to work with, and can be used to see inside of stuff. 

It's a relatively new field of application because it depends upon laser techniques for its generation; the frequencies are too high for electronic processes, but [new techniques are always under development.](http://ns.umich.edu/new/releases/22188-new-t-ray-tech-converts-light-to-sound-for-weapons-detection-medical-imaging)

[Terahertz and Cultural Heritage Science: Examination of Art and Archaeology](http://www.mdpi.com/2227-7080/4/1/6/htm) goes into great detail, beginning with analysis of hidden layers in art, where it is used to explore underneath the visible surface:

[![enter image description here][1]][1]

In section 4.1 they give examples of scanning through multiple pages at the same time. 

> **4.1. Historical Documents** Often, historical documents on papyrus as well as on parchment and on paper cannot be read for a number of
> reasons. Sometimes the fragile sheets just cannot be separated because
> they are stuck together as a result of deterioration and damages. In
> other cases, the sheets have been reused as supports or covers for
> newer documents. There is considerable interest in reading this hidden
> information and a number of techniques have been tested to pursue this
> goal while preserving the documents. So far, X-ray computed tomography
> [66] has been the most successful method. Since THz-TD imaging was
> already evaluated for the inspection of postal envelopes [67], it was
> also tested with encouraging results for stacked papyrus layers
> written with carbon black ink [68]. Recently, even more successful
> results were obtained with a new and more sophisticated THz method
> called tomosynthesis [69] which was applied to image pencil writing on
> a stack of 50 paper sheets. THz tomography has been applied to resolve
> text on both sides of a single papyrus sheet [70,71]. THz was tested
> also for those cases where writing is obscured by stains and other
> inks in old purchament manuscripts [72] and it seems successful to
> characterize and evaluate conservation of iron gall inks [73] and
> parchment [74].


So with a properly calibrated T-Ray camera, and the tomographic software for image reconstruction, one can indeed scan through the pages of a book, top to bottom, never having to open the book - and without any hazard for the librarian.  Just so long as the book isn't too thick.

**Update:** Demonstration of reading through nine pages - [MIT News: Judging a book through its cover](http://news.mit.edu/2016/computational-imaging-method-reads-closed-books-0909).

  [1]: https://i.sstatic.net/e7YOz.png