Purchased the 3-shelf and 4-shelf bookcases to put side by side.  Shelves do line up horizontally, but not if you look downward.  Bottom shelves are about 3/4" off.  That was disappointing, but we hope that no one is looking!  Also, the end framing on the 4-shelf unit comes in 2 pieces which needs to be screwed together.  We were a little concerned, but it lined up almost perfectly.  For the price, that was surprising that each end piece wasn't a single piece.  However, after looking at this in our house for a few weeks, we are more happy than not though.  Both pieces are very sturdy and look nice.  We did not have to anchor the bookcases to the wall.  And it did not take long for two of us to put it together.