Run farther up the road to the library; there, you can further your knowledge.
Further [THINK: figurative (U)]
Farther [THINK: physical distance (A)]
“Do you use farther and further interchangeably? You’re not alone….”
“The widely accepted rule is to use farther to discuss physical distances, as in He went farther down the road.”
“Further should be used for figurative distance or to discuss degree or extent….or advance, a project…. Further also has an adverbial sense of “moreover; additionally…”
[read more about it on blog.dictionary.com]