This IKEA hack utilises the wasted space in the attic walls by placing STUVA units into the recesses. You'll want to measure everything (twice), naturally, reinforce the backs of the units and make sure everything is level.
After cutting the wall opening, prepping the bookcase/storage units, and installing the unit (pre-drill the back bottom of the lower unit for easier installation), you can add moulding around the system and marvel at your space-saving solution.
This concept may also be adaptable with other shelving/locations: you could inset shelves beneath stairs, for example, or set shallow cupboards between wall studs, perhaps.
Space-Saving STUVA Storage Closet and Shelf Inset Into Wall [IKEA Hackers]