A great book but may be all Greek to readers (7)

I believe the answer is:

classic

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

'a great book' is the definition.
'classic' can be an answer for 'book' (thesaurus). I am unsure of the 'a great' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for classic that I've seen before include "Race, eg Derby" , "Very great work of art" , "First-class" , "Major long-standing sporting event" , "big race" .)

I've seen this clue in The Globe and Mail.

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