In this case, ever doesn't really mean much at all - it's effectively functioning as an intensifier. We could, for example, rephrase it as...
This was the nearest I had been to her at any time up until that point.
...but obviously semantically the italicised clause doesn't really change the meaning of the statement.
Note that there are other contexts where superficially similar use of ever does affect meaning...
1: "Have you brushed your teeth?"
2: "Have you ever brushed your teeth?"
...where #1 almost certainly implies "...in the very recent past" (probably, in the last few minutes). But #2 is unlikely to be used with literal intent in any normal context (it would probably only be said facetiously).
If I said, "Have you ever been to Spain?", I likely mean that literally: have you, at any time in your life, been to Spain? Sure, "Have you ever brushed your teeth?" is probably not literal. But that doesn't mean that "ever" is simply an intensifier. It's more like hyperbole here. – Jay – 2013-04-22T15:12:00.170
@Jay: What, you seriously mean you'd classify Harper Lee's usage as "hyperbole"? I think that might be something of an exaggeration! :) – FumbleFingers Reinstate Monica – 2013-04-22T15:19:01.000