Something for the mind that's offered in a tablet? (1-4)

I believe the answer is:

e-book

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'tablet?' is the definition.
I know that 'tablet' relates to this answer.

I don't understand how the remainder of the clue works.

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(Other definitions for e-book that I've seen before include "Downloaded volume" , "Electronic reading material" , "Online publication" , "Non-paper publication" , "Volume in electronic form" .)

I've seen this clue in The Telegraph.

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