Lowercase s

Turn Caps Lock off, then press s without Shift. If S appears instead, check those two case controls. On an onscreen keyboard, switch to lowercase before tapping the letter.

Why does an old book show an f-like s?

It may be long s, written ſ, rather than f. Historical printing used this elongated form of the letter. It stands for s, despite looking like f.

For a modernized transcription—a typed version using today's letterforms—replace ſ with s. To keep the original letterform, enter ſ itself; changing the font of s does not change which character you typed.