I think it depends on what you are talking about. If you mean "lettré" as an adjective for a person (the most common use I think), well-read is to my mind the best option because it suggests both reading and education.

Here is a useful dictionary:

http://atilf.atilf.fr/

If you want to look at the entry for "lettré" you will have to type it into the dictionary. I tried to insert a link but it does not work...

I hope this helps.