For an alternative method, add a cabinet. A standard kitchen cabinet can be used between the bookshelves if you would rather have storage than legroom. This will also serve to give the island a more solid look and can be used to hide dishwashers and other appliances from the main view of your kitchen.
You will need your cabinet and bookshelves to be the same height so that the countertop will go evenly across all three pieces. The easiest way to do this is to have shelves which are slightly shorter than the cabinet and add legs. You will also need the cabinet to be no deeper than the width of the bookshelves.
The countertop will then need to be the length of the depth of both bookshelves, plus the width of the cabinet, plus a little extra to create the lip of the counter. The width of the countertop will be determined again by the width of the bookshelves.
Attach the countertop to the cabinet and bookshelves by first screwing through the inside of the cabinet into the backside of the bookshelf (preferably at the sides as before, but possibly through the lower and upper horizontal sections if they can be reached). Then screw through the inside of the cabinet into the countertop above, again being careful of the length of your screw.
DIY Kitchen Island Tutorials ‘BHG‘ used two Hemnes Ikea sofa tables and a laminate top to make a kitchen island with lots of storage. Don’t forget the wheels for portability! Full tutorial for this project. From ‘Yellow Brick Home‘, make a kitchen island from a dresser with this step by step tutorial. You could use a thrift store.