Read enough old books and you’ll probably encounter more than once an unusual letter that looks like an f without a crossbar — ſ — where an s ought to be. It’s not an f, it’s a since-abandoned form of the lowercase s called the long s, and it was used at the beginning or the middle of a word; the s we know today, the short s, was only used at the end of a word or where the s would be doubled, e.g. miſtreſs, diſtreſsing. (There were some exceptions to this rule, of course, which BabelStone explains in a long post about the long s’s usage.)
The long s has a long history but it’s all but forgotten today. Now it haunts archives and rare book rooms, waiting to entrap unwary graduate students who dutifully recopy it as an f. (This is wrong: write it as an s.) It lived on in Fraktur long after it disappeared in Roman type, and survives today as part of the German letter ß, which is essentially (or eſsentially, or eßentially) a ligature of ſs, and in fact “ss” is used for ß where that letter is unavailable.