you could use any number of glues, E6000 is very clear but has serious fumes. MAC is considered the very best but is really expensive. Any white PVA like weldbond works but takes some patience to wait for it to clear - use it sparingly

For this project, I would use MAC glue. It is so expensive that I quit buying it, but for this project I would buy one more bottle. I would put a thin unbroken line of the glue all the way around the edge of each piece of glass.

Thank you both for the responses. Will do as suggested.
Diane

You can also do a technique called plating where you would actually solder the layers together. I made this piece that has layers in front and behind. The palm fronds are the layer in front and there are layers behind in the water (to add color and depth you can't get from one glass), the bird, and all shadows. I have more designs on my site.