Question

This is the most difficult book. [use: as difficult as]

Answer 1

no other book is as difficult as this

Answer 2

No other book is as difficult as this .

DETAILS :

This is the most difficult book .

This means that no other book is more difficult than this.

Hence the sentence can be written as :

No other book is as difficult as this .

Here I have used " as difficult as " .

This is the preferred answer .