Hi, kitty914,

I don't usually post in this forum, but since no one else has weighed in, I'll share some ideas. You already have some experience with furniture, but if I'm not making myself clear, let me know.

1) First thing I'd do is build an open-front box under each ikea bookcase, to set the top of the case 1/2"-3/4" above the bottom of the mantel. (Enough to nail into--see step 3.) If that leaves enough room for storage underneath, that's bonus space, or if not, I'd cover the front with a plain panel. (Or a panel to suit your style.) It could even be a secret hiding place for valuables--I won't tell anyone. ;)

2) I'd then build two 3-sided boxes (columns)--one to fill in the space between the brick and bookcase on each side. You could make the boxes 4-sided, by why waste material that will be against the wall anyway? Wood scraps could be used as braces, top and bottom, on the backs. A half-column trim piece could be used on the fronts for interest--fluted, turned, or plain--again, to suit your style, or the style of the house. Bring them out even with the front of the mantel, or set back from the fronts of the bookcases, depending on the decorative element used on the fronts. Shoe molding would finish off the bottom, and I'd attach the boxes to the sides of the bookcases (from the bookcase side), countersinking the screws, which will then be covered by books.

3) Cover the whole top with a board that is 1 1/2" deeper the bookcases (for overhang), and wide enough to reach from wall to wall. A new trim piece as thick as the mantel (and wall to wall in length) can unify the front of the mantel and both bookcases. Set it back, leaving a slight overhang. I think you could use finish nails for these pieces, since a couple of good hammer whacks would break them loose for dismantling and moving. Small holes could then be filled, and touched up with paint.

4) Paint or stain all the wood to match. (Or paint before it's assembled, and touch up later--no paint on the brick that way.)

Add artwork, stand back, and admire.

I can't tell how deep the mantel is in relation to the bookcases, but I think you'll get my idea. It seems that this could all be taken apart, leaving barely a trace. And if you've already started on another design--good luck and enjoy!

BTW, if you need any more ideas, gardenweb's Home Decorating Forum is great. Lots of talent there.

Here is a link that might be useful: mantel/bookcase ideas