How to mount a corner shelving piece ?

Here are two plans. The first one is the most conventional and will hold the most weight.

Plan 1.

Use triangles of wood as braces. Two per shelf. The length of the right-angle sides of the brace should be at least as long as 3/4 of the span from the wall to the edge of the shelf. The max length would be equal to the span from the wall to the edge of the shelf. Drill a hole through the brace edge about 1 inch up from the bottom. The direction of the hole is horizontal from the hypotenuse surface toward the wall. Put a bead of wood glue along the side of the brace which will touch the wall. Place the braces about an inch in from the edges of the shelf. Screw through the hole in the braces into the wall. Drill two holed in the shelf so that you can screw downward through the shelf into the braces. You can use glue instead of screws to hold the shelf on.

Limit the weight of stuff you place on the shelf unless you add a third brace per shelf. The third brace would be deep into the corner where the two walls meet. In that place you are sure to hit a stud with the screw that holds the brace. That third brace can be a lot shorter. It could be as small as a two inch square block. That third brace will hold any kind of weight that you could reasonably want to place on that shelf.

Plan 2.

Screw three blocks of wood to the wall to hold up each shelf. Each block should be a two inch cube, approximately. Drill one hole through each block to mount it to the wall. The corner block will be screwed to a stud because there must be a stud in the corner. That will hold the weight. The other two block will be screwed to the wall as far from the corner as possible, less an inch.

Drill three holes in the shelf so that you can screw down through the holes into the blocks, holding the shelf firmly. No glue is needed in this design but the shelf must be screwed down onto the blocks.

Mayan

I'm toiling to visualise how the wall looks but recently I bought a large LCD tv and it wasn't going to fit exactly in the middle of the wall because of the beam location. What I ended up doing was buying a big sheet of inch thick ply wood and cut out a space in the wall so the wood could fit in then I secured it to the beams. After that I plastered it and wallpapered and you wouldn't notice the difference. This gave me the solid surface I needed to mount the tv where I wanted. I'm not sure how this would work with a lath and plaster wall as I know it gets very messy when you start messing about with it.

Besides decorative L brackets I can't make any other suggestions unless you're only putting SUPER light things on these shelves, in which case you can use a serious adhesive like Gorilla glue. But you wouldn't be able to hold more than a pound on them, tops.

Here is a tutorial on corner shelf building with pictures.Enjoy.