Happy Home Handbook
The Happy Home Handbook is a manual containing lessons on home design that the player can study to unlock new features, such as the ability to place furniture on the ceiling, in Animal Crossing: Happy Home Designer. On the ninth in-game day, after the first public facility has been built and designed, Tom Nook gives it to the player as a gift. It can be accessed by sitting at the desk where the game can be saved – located on the first floor of Nook's Homes – and then choosing to study the handbook. In total, there are fourteen lessons, and all of the lessons (excluding the first one, which is offered for free) require a payment of one, two, or five Play Coins to be unlocked.
The lessons also unlock the thing that the lesson taught you.