Puzzle ZNTH 

Workings

Hint
A little algebra might help.

Answer
He will be 35 in 14 years.

Reasoning
He is currently 21.

If X is his current age, then:
  X = 14 +  X
           ———
            3
so

   2X = 14
  ———
   3

and

  2X = 42

So

  X = 21

He is currently 21 and his favourite book is currently 14 years old, when it is 28 years old (double its age) he will be 35.