farther v. further

Use farther to refer to literal distances and further when describing metaphorical advancement, or to mean ‘additional’ or ‘moreover’. So you can drive farther while pursuing further adventures.

In some cases, either word can be used. For instance, ‘reading further in a book’ has the sense of having taken in more content (abstract), but ‘reading farther’ focuses on more pages having been flipped (literal).

Grammar Girl on farther v. further

Answers.com usage note on farther v. further