It didn't take long before I locked my search down on Ikea Billy Bookcase. Here are what you need and a step-by-step guide to hack the Ikea Billy Bookcases into a sensational wall-to-wall built-in unit:
Depending on the length of your wall, you will need 5 x extra deep (15 3/8") Ikea Billy Bokocases. I chose the 15 3/8" one over the standard 11" for more shelf space. You really can't put much on the 11"-shelf other than books.
To complete a more "built-in" and integrated look, you will also need crown moulding, flat base moulding as filler for the in-between space, any paintable caulking to touch up nail holes and gaps, and of course a pneumatic nail gun and air compressor.
Our wall is 171" in length. Ikea Billy Bookcase each is 31.5" in width. After some simple math following this formula: (Wall Length - 31.5 * Quantity) / (Number of Fillers) = Width of the Filler (or Flat Base Moulding).
Our calculations lead us to 2.25" gaps in between the Bookcases. You can also get a MDF sheet to cut them into desired width. I prefer the Flat Base Moulding as it comes in a convenient 8' length. It's also thick (5/8") enough to take nails in.
You will then punch nails at a downward angle from the bookcase to secure these filler mouldings in place. Add the crown moulding over the top, patch up the nail holes and gaps in between, and you are done.
Not counting the time we spent on assembling the bookcases themselves, the entire hack project took us about six hours. The toughest part was by far to level each bookcase unit, after that, it's only a matter of cutting and securing mouldings in place.