A mahogany bookcase flatpack can be made at a workbench (oak workbench or better) in the workshop of a player-owned house. This requires 40 Construction, uses 3 mahogany planks, and gives 420 experience. The flatpack can then be used on the bookcase space hotspots in the parlour, quest hall, or study, while in building mode. This requires no Construction level and gives no experience.
The cost of making and selling a mahogany bookcase flatpack from scratch gives a profit of: -3,871
After completing the hard Fremennik achievements, they can also be traded to Advisor Ghrim in exchange for 507 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.