Crouch takes them in that order and goes through their careers chronologically, singling out the decisive moments in these twenty games, devoting several pages to each game. The styles of Lasker and Petrosian were widely contrasted. Petrosian usually was content to draw with the black pieces, but sometimes he had to fight. This book is very interesting and full of good advice, although I think it would have been better still had Crouch broadened his treatment of defence to include one or two other players. Korchnoi, for example, has won countless games by accepting pawn sacrifices that most grandmasters would reject and finding the flaw in the attacker's plan.
Most players probably prefer attacking to defending.
Occasionally, as in his analysis of the Hamdouchi-Lautier game from Cap D'Agde 1994, he really looks into the depths of a position but too many of his comments are superficial. Also, attack for Christiansen in this book largely means mating attacks against the enemy king with other types of attack not really discussed. The opening chapter about his development from schoolboy beginner to grandmaster is interesting enough but hardly belongs in a book on this subject. Finally, the book winds up with a countdown of Christiansen's 11 favourite attacking games of the 1990s and, as a postscript, a late game by Tal. Openings books, of course, are ultimately about how to acquire good positions in the first place - or avoiding getting into bad ones.