These are the extras - the soundtrack by Iede Saudek and Thomas Krze, the preliminary works by Camila "Bura" Gormaz and a hi-def illustration by Lia Margot. After downloading the DLC, open up Invisible Apartment's folder to find the "extra" directory.

If you downloaded the latest version of Invisible Apartment, than you'll see a button saying "extras" on the main screen and on the launch screen.

Tracks included:
Iede Saudek - Gnossiene 1
Thomas Krze - Dreamin 1
Thomas Krze - Dreamin 2
Thomas Krze - Gymnopedie 1
Thomas Krze - Colibri
Thomas Krze - It's not easy to say goodbye
Thomas Krze - First snow xmas 2
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If you enjoyed Invisible Apartment 1 than you will also like the other parts of the series:

http://store.steampowered.com/app/543230/
http://store.steampowered.com/app/543220/
http://store.steampowered.com/app/532060/