The only way to delete something in map maker is to press Ctrl+Z, but this only works once so you'd have had to have done it after you placed the bookstore down - so unless you saved your map at different intervals along the way, your screwed.
Tip is to save your map everytime you get something major done, like connecting the outisde walls, filling in tile areas or adding objects - that way if you make a mistake you can't undo you can open up and earlier version and redo what you need to.
