Crossword Genius

The keys to knowledge are nature and time, he thought (4)

Ross

I believe the answer is:

book

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

'the keys to knowledge are nature and time he thought' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'he thought' is the wordplay.
'he' becomes 'Bo'.
'thought' becomes 'ok' (I can't explain this - if you can you should give a lot more credence to this answer).
'bo'+'ok'='BOOK'

Can you help me to learn more?

(Other definitions for book that I've seen before include "Reading volume" , "Quick work" , "Make a reservation" , "Take down name of offending player" , "Rebecca or Emma maybe" .)

I've seen this clue in The Telegraph.
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