Crossword Genius

Taking fifth of timber reserve, this spares trees (1-4)

Ross

I believe the answer is:

e-book

I'm a little stuck... Click here to teach me more about this clue!

'taking fifth of timber' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'reserve this spares trees' is the wordplay.
I cannot really understand how this works, but
'reserve' could be 'book' (to book something is to reserve it) and 'book' is found in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for e-book that I've seen before include "Literature from the internet" , "Electronic reading material" , "Non-paper publication" , "Downloaded volume" , "Online publication" .)

I've seen this clue in The Guardian.
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