What is the correct glue to use to adhere wood to glass?

A silicone glue is best for this project. Something like this: Silicone Adhesive

My hubby loves JB weld! I got this definition from google....so maybe it would work on your project:

'J-B Weld is a two-part epoxy adhesive (or filler) that can withstand high-temperature environments. J-B Weld can be used to bond surfaces made from metal, porcelain, ceramic, glass, marble, PVC, ABS, concrete, fiberglass, wood, fabric, or paper.'

Dollar Tree sells a glue called the Original Super Glue that says it will do this. I’ve glued plastic to wood with it.

It takes a long time to dry and requires squeaky clean glass.

Clear silicone will also work. You can buy it at tthe hardware store

E6000 glue.

E6000 is my 'go-to'!

If the letters are not too big or heavy, even Elmers glue will work.

Hello-I think E6000 would be a great choice. Its at walmart and craft stores-

Ive used it to flatten glass marbles and faceted glass plates on woodn glass windows to make decorative hangings.

I might suggest painting both sides of your wooden letters. This is a nice crystal clear glue.

Please have a look at product information to see if it might meet your needs.

http://eclecticproducts.com/products/e6000/e6000-craft.html

e6000 glue is amazing stuff. It's not immediate so you might have to tape them in place while they dry.