Using Ikea Billy Bookcases as a starting point for building custom bookshelves on a wall or around french doors in your home is a great option. It is the ultimate Billy Hack!
What you buy from Ikea will depend upon the dimensions in your space. I bought 2 bookshelves with extenders, plus one skinny bookshelf to go over the french doors.
I personally love the look of bookshelves with painted backs. I think it makes the built ins look much more custom.
Using the provided hardware from Ikea, attach the bookcases to the wall to secure them. I also built a small platform for them to sit on so that the baseboard would fit on the bottom of the shelves.
Now that the built ins are secure, add trim around the front of the shelves to make it look built in. Add baseboards around the front as well. Add crown moulding to the top. Caulk all of the seams, then paint!
Now for the fun part! Style the shelves with your favorite books, pictures, and decor.