Fowler's English Usage, pg. 189:

'Farther, further: The history of the two words appears to be that "further" is a comparative of "fore" and should, if it were to be held to its etymology, mean more advanced, and that "farther" is a newer variant of "further", no more connected with "far" than "further" is, but affected in its form by the fact that "further", having come to be used instead of the obsolete comparative of "far (farrer)", seemed to need a respelling that should assimilate it to "far" ... The OED says: 'In standard English the form "farther" is usually preferred where the word is intended to be the comparative of "far", while "further" is used where the notion of "far" is altogether absent; there is a large interemediate class of instances in which the choice between the two forms is arbitrary.'"'

Fowler adds that most people these days seem to prefer "further".