A few of the comparatives and superlatives in English do not follow the usual pattern. Here is a list of common exceptions.
|late||later||latest or last|
The comparisons for well apply to both the adjective meaning "healthy" and the adverb meaning "in a good manner."
For more on how to use some of these see the Common Mistakes section on good/well and bad/badly. Also see Common Mistakes section for the difference between further and farther and between littlest and least.