It’s the “AE” in AEsop, amongst other things.
The glyph you see in the illustration is one you’ve likely met before but never caught its name. Allow us to introduce you to the aesc (pronounced, quite counter-intuitively, “ash”). You’ve seen it in the past in words such as Æsop, you once saw it in words like anaesthesia (anæsthesia) and aesthetic (æthetic), and it is no loger seen in the word medieval (which used to be spelled mediæval)
Bringhurst tells us that the letter approximates the Swedish ä sound, which, presumably, isn’t much unlike the English short e (or the German ä, for that matter) and it is still used in Danish and Norwegian. Its English point of entry was apparently words from imported from the Greek whose spelling began with alpha-iota (αι). Therefore, for pedantic correctness and authenticity in old quotations, this ligature is still included, and rather popular.
Source:The Elements of Typographc Style, v2.5, Robert Bringhurst
