The Longest Journey
The Longest Journey
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In the living room of the Border House, to the left of the corkboard, there is a small, unmarked wall area that when hovered over will reveal a caption for a "Painting". Clicking it will instead cause April to walk to the adjacent right wall where the unseen painting is hung and comment:

April: "It's a reproduction of an early Lebowski. Not a particular favorite of mine, but, there ya are."

During the cutscene "Vision in the Living Room" which is only played if April has chosen to not work the evening shift at the Fringe Café, a shot of the painting depicting an abstract rendition of a woman's head can be seen. This painting appears to be a caricature of American actress Julianne Moore, who appeared one year prior to The Longest Journey's release in the 1998 film "The Big Lebowski" in the role of artist Maude Lebowski, suggesting that the painting may be a self-portrait.
Contributed by MehDeletingLater
Outside the Mercury Theatre, a poster can be found for a fictional film called "A Welsh Ghost Story" that April claims is "a classic", and is written and directed by Ragnar Tørnquist, the game's creator. This film was actually based on a real work-in-progress screenplay with the same tentative title by Tørnquist that would later be uploaded online in 2014 under the name "In the Dark Places".
Contributed by MehDeletingLater
In the living room of the Border House, hovering the mouse over the back part of the horizontal stand the TV is being held up on will display a prompt for a "Hidden button". Clicking it will cause the TV to play the first teaser trailer for The Longest Journey.
Contributed by MehDeletingLater
The Book of Secrets is a showcase of voice acting outtakes, unused music and concept art that can be unlocked by beating the game and is accessed from the title screen. However, there is a secret way to unlock it earlier on in the game.

Spoiler:In the hallway outside April's bedroom, a leaf from an Organic plastic plant can be collected. This leaf can then be used on the Flowerbed in The City Green, triggering an animation of April planting the leaf next to it which immediately sprouts a large flower. She will then comment about hearing a voice stating that a "Book of Secrets" was now open, unlocking the bonus content.
Contributed by MehDeletingLater