Longest Journey, The Easter Egg - Book of Secrets

1: start game
2: after you leave your room for the first time take a look at the plastic plant in the corner. Take a leaf.
3: continue play of game as normal until you are invited by Azabrax? (spelling) the creature who can see through time.
4: go to his house and plant the leaf in the garden beside the flowers, it should reference to it as fertile ground. If it the correct spot a plant will grow and you'll get a comment about the book of secrets has opened.

5: go to the title page and you will find an extra option to look through "the book of secrets"

enjoy

P.S. if you look at the large plant in the garden there is a reference to Audrey II (feed me seymour)