There are many books suitable for beginners that teach about
zazen. Understand that zazen is a practice. The best way to learn
about zazen is to do it--not just to read about it. From that
perspective, the best book for beginners that teaches not only some
of the theory behind Zen Buddhism but also clearly instructs how to
begin a zazen practice at home is still Philip Kapleau's THE THREE
PILLARS OF ZEN.
.