Summarize:

These should be roughly counter-height or just above. They should be sturdy and preferably deeper than the standard bookshelf. You can paint them beforehand if you want them to be a different color. Measure their depth and width. Decide how long you want the countertop to be. It should be at least as long as the depth of both shelves plus a little extra to create the lip of the counter but can include up to four or five feet between the bookshelves. Next, determine the width by taking the width of the bookshelves and adding a little extra to create a lip. Once you know the dimensions you need, you can purchase or make your own countertop. Make your own by purchasing medium density fiberboard (MDF) or you can go to your local hardware store and purchase any number of materials cut to the exact size that you need.  Butcher's block is a popular option as it is cheap, easy to clean, and great for use in kitchens.  Granite may also be an option but since slabs are so heavy, you will need less space between the bookshelves in order to be sure it is properly supported. If making your own from MDF, you can paint it to create a more table-like appearance or you can laminate or tile the surface to use the island for food preparation. With the shelves at the ends facing outward, place the counter on top and attach it to the shelves with brackets. These brackets will need to be screwed into bookshelves at the edges where the wood is thickest and then into the countertop, being careful not to use screws that are too long and protrude through the surface. Special considerations will need to be taken if using a granite countertop, as you cannot simply screw into the stone. Consult with your local hardware store before attempting this type of counter. If using MDF you can paint, tile, or laminate the countertop based on your needs and preference. Hooks can be bracketed or glued to the outside of the book shelves to hang kitchen towels. Depending on the type of bracket you chose to use, it may be possible to suspend a bar and hooks between the brackets in order to hang pots and pans. Keep in mind, however, that this can quickly become too heavy for the brackets, so don't hang too much. 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 that 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.
Get two identical bookshelves. Figure out the dimensions of your countertop. Purchase or make a countertop. Attach the counter to the shelves. Add any desired finishing touches. For an alternative method, add a cabinet.