Actually, in my HarperCollins reprint (1998), the characters do go "farther up and farther in". Which I find irritating, because it's not what Lewis wrote — all the early editions (and indeed the 1980s edition I grew up with) definitely had "further"!! I have no idea why they changed it, when either one is perfectly acceptable and I think Lewis did have enough command of the English language to choose for himself which words he wanted... (And since he did all his schooling in the UK rather than the US, he wouldn't have had Strunk and White as a reference anyway.)

Now I'm interested too!
My Concise Oxford Dictionary does cross-reference the two words, but gives similar differences of meaning to ecurb above. [My old copy of Strunk & White has long gone, but I had no idea it wasn't printed in England, as it was in use in my very UK style high school and university, here in NZ]

A straightforward book I currently have for ESOL teaching is Michael Swan, Basic English Usage; he says both further and farther are used of distance, and there is no difference of meaning, but then in brackets, "Only farther is used in this sense in American English". He goes on to say that only further, not farther, can be used to mean 'extra', 'more advanced', 'additional'. Was this a change that crossed over from the American editions? If so, WHEN did it happen?

My 1973 Puffin paperback is the only copy I have, and it says "Farther" both in the chapter title and in the text. It is an English edition, and less than 20 years since the first edition. It can't be an American edition change, since Puffin has printed it in UK.
In Paul Ford's 'Companion to Narnia', he quotes "further" only. (My edition is 2005).

Can anyone else cite English edition printings, giving the year and which word was used?

I didn't know that some editions edited "further" into "farther". Boo! Hiss!

I boycotted the Harry Potter series for years because the American versions "translated" English into American English --"Philosopher's Stone" became "Sorcerer's Stone" in the title of the first book, which I thought was obnoxious. American children are fully capable of learning what a philosopher's stone is. (I finally gave in and read Potter because I like children's fantasy literature.)

It seems to me that "further up and further in" hints at something beyond mere distance -- like going further in to one's "real" self, or further into some Platonic "reality". I have no idea if that was Lewis's notion (although since he was a word smith and a literature professor I don't doubt that he was familiar with the subtle distinctions in meaning of the two words.)

I have a Harper Collins paperback version now, and I was weaned on the hardbacks that were probably the original U.S. publications. I can't verify it, but I think they stuck with "further".

My 1973 Puffin paperback is the only copy I have, and it says "Farther" both in the chapter title and in the text. It is an English edition, and less than 20 years since the first edition. It can't be an American edition change, since Puffin has printed it in UK.
In Paul Ford's 'Companion to Narnia', he quotes "further" only. (My edition is 2005).

Curiouser and curiouser! The one I grew up reading was a 1980-something Fontana Lions edition (this cover), printed in the UK as far as I know, and it most definitely had "further".

It seems to me that "further up and further in" hints at something beyond mere distance -- like going further in to one's "real" self, or further into some Platonic "reality". I have no idea if that was Lewis's notion (although since he was a word smith and a literature professor I don't doubt that he was familiar with the subtle distinctions in meaning of the two words.)

Totally agree!

I have a copy of the book from 2001 that uses further. I believe it was published in America.