The English language doesn't always make a whole lot of sense. The word "subtle" has an inexplicable b in the middle of it, "knife" is spelled with a k...and a single letter can totally change the meaning of a word. So when you're writing, are you getting farther along in your book -- or are you getting further along in your work? Maybe you're doing both, and either way you'd better know the difference.
Farther vs. Further
It's an epic word battle, and one I personally struggle with every single time. No matter what I'm writing, the minute I want to use further or farther, I'm forced to stop what I'm doing and look up the differences between them. It's just one little vowel, but it makes a big difference if you end up using the wrong one. In fact, it changes what you're writing entirely.
Farther refers to an actual physical distance that you can measure with a ruler (or, you know, whatever). A car drives farther down the road; you will get farther on your trip to the beach; you might be asked to come farther into a room. You can walk farther. You can never walk further.
Further denotes a figurative distance, and maybe that's where I run into my own personal writing troubles when the word crops up in a book. The word is used only metaphorically, not to describe an actual measurement of length, space or distance. For example, I will get further along in this post if I keep typing it. This post isn't going to cover a physical distance, and I'm not moving from Point A to Point B, so I'm using that word metaphorically. If I sit around wasting a bunch of time, I'll be getting further away from my blogging goals. I'm not actually going anywhere; the distance isn't an actual distance that you can measure.
It gets confusing, because it isn't always easy to tell when a distance is actual and when it's just a metaphor. When you're writing with further and farther, look at what you're describing in the sentence and ask yourself if you can do it without getting off the couch (I can get further away from going to sleep while sitting on my couch and playing video games, but I can only get farther away from the TV if I get up and move my body). If you can't, you need to go farther. If you can, you're going further.