From the Online Content - Tenth Anniversary Edition-- Updated with New Material.
January, Vitality: boost energy
February, Marriage: remember love
April, Parenthood: lighten up
May, Leisure: be serious about play
June, Friendship: make time for friends
July, Money: buy some happiness
August, Eternity: contemplate the heavens
September, Books: pursue a passion
October, Mindfulness: pay attention
November, Attitude: keep a contented heart
December, Happiness: boot camp perfect
The Happiness Project manifesto
A conversation with Gretchen Rubin
Excerpt from The Four Tendencies
Suggestions for further reading.