I'm also wondering that... Would it work as bookshelves 12 feet long?

There is a bracket you can buy that is a steel plate with a rod welded to it that screws to the wall. Similar to an ikea shelf

Any idea where one can obtain this bracket? We're trying to do the same thing, but haven't been able to find anything large or strong enough.

You can attach shelves like this using metal dowels. You attach the dowels to studs in the wall and drill holes in the back of the shelves the same size and distance apart. Using an epoxy to secure them, you slide the shelves into the dowels, and voila.

I just looked up Lee Valley Tools, mentioned above, and their bracket will only support a shelf up to 8" deep, which will not be adequate for kitchen shelves.

We added a couple floating shelves ourselves.
These are about 1 3/4" solid pine. I glued and bolted them to 1/2" plywood and also added hardwood behind the plywood to strengthen everything. I cut out the drywall and screwed shelves/plywood to wall. Tiled around shelves over plywood.
The whole assembly is very strong.

Thank you for describing how you installed your floating shelves. I'm looking at doing that and this is useful information!

What is depth of these shelves please?

What size tiles are those? 8X2 or 6X2?