The hallway in our new flat is quite long (5m), perfect for a long book shelf. Getting a full size shelf like Billy or Ivar would be expensive and would have given us a shelf which is deeper than what we needed for books. A friend was selling of her large Expedit shelf in white but it is 38 cm deep and only 185 cm wide. After a joke of my wife, we settled on splitting the shelf into two halves along the long axis to get a 370 cm wide and 19 cm deep shelf - just perfect for all the books and stuff in our flat.
We took the used Expedit apart and basically cut each piece on a circular saw at my dads and bought a big bag of pegs (you'll need twice as much for the two halves) to fix all the 30 vertical boards.
As in our particular case, the full 370 cm width would block the entry door, so we needed to make is shorter. But removing a single column would make it asymmetric where as removing two columns would make it to short. After a while, we achieved symmetry with the 9 columns by removing one from box halves but putting one back in between them.
See the pictures... :)
Step 1: Final Installation
After posting this instructable, we've been asked by an anonymous reader, whether the shelf is stable to hold all the books and other stuff. Sure, it is. As all boards are kind of "solid", cutting them in half did not change its stability. As with most shelves, you should fix them to the wall at the top corners (as IKEA tells you). We did at 3 points and it really holds. I just tried climbing on it without problems.. :)