Grammar of "No book is really worth reading at the age of ten which is not equally–and often far more–worth reading at the age of fifty and beyond" [duplicate]

Solution 1:

The essence of C. S. Lewis's quote is "If any book is worth reading when you are 10, then that book is also worth reading (and sometimes even more worth reading) when you are 50 or older."

(To give context, this seems to be a response to the criticism that his "Chronicles of Narnia" series of books were "only children's books.")

No book is

{really worth reading (adjectival complement)

at the age of ten (adverbial phrase modifying “really worth reading”)} adjectival complement of “no book is”

{which is not equally worth reading at the age of fifty and beyond.} Relative clause modifying all of No book is really worth reading at the age of ten

– and often far more – Parenthetical emphatic.

The corollary of this negative argument is the positive:

Any book that is worth reading at the age of 10 is also worth reading at 50-plus.

Solution 2:

Start with the quote.

No book is really worth reading at the age of ten which is not equally – and often far more – worth reading at the age of fifty and beyond.

Remove the elaboration between the em dashes.

No book is really worth reading at the age of ten which is not equally worth reading at the age of fifty and beyond.

Remove the elaborative propositional phrases.

No book is really worth reading which is not equally worth reading.

And there you have it. You've arrived at a sentence that makes little sense due to insufficient information. So you can assume that the specifier "which" relies on the elaboration offered by the prepositional phrases. No book is worth reading at the age of ten which is not equally worth reading at the age of fifty. Or, in other words, every book worth reading at the age of ten is also worth reading at the age of fifty.

Solution 3:

No book is really worth reading at the age of ten which is not equally – and often far more – worth reading at the age of fifty and beyond. -C.S. Lewis

Take out modifiers really and equally, and supplement and often far more, move the relative clause which is... to right behind the noun it is modifying book, and we get:

[No book which is not worth reading at the age of fify and beyond] is worth reading at the age of ten.

Take out no and not as well since they cancel out,

[Books which are worth reading at the age of fify and beyond] are worth reading at the age of ten.

Switch the VPs of the relative and main clauses, and we are back to something very close to the original meaning, and have gotten rid of the the post-posed relative clause.

[Books which are worth reading at the age of ten] are worth reading at the age of fifty and beyond.

Add the modifiers and supplement back in,

[Books which are really worth reading at the age of ten] are equally - and often far more - worth reading at the age of fifty and beyond.