Brain Training Puzzles: Difficult Book 1 : Brain Training Puzzles: Difficult Book 1 is part of a series of six books designed for you to make the most of your mental potential. Each book in the series contains 150 puzzles, one per page. The puzzles increase in difficulty level as you work through, providing a constant challenge: keep your brain fit! There are many different puzzle types, ensuring a challenge for everybody, from the casual solver to the serious puzzle nut! Difficult Book 1 is filled with tough puzzle types and challenging puzzles.