They’re using Thorn Edd, the single character that represented the Th sound in old English (still used in Icelandic).
It’s a harmless little quirk in their own writing, although editing the title of a book to include it seems pretty silly.
You are correct. In my defence:
In Old English, ⟨ð⟩ (called ðæt) was used interchangeably with ⟨þ⟩ to represent the Old English dental fricative phoneme /θ/ or its allophone /ð/, which exist in modern English phonology as the voiceless and voiced dental fricatives both now spelled ⟨th⟩.
So I looked it up. The character they used is “eth”. Which historically was used interchangeably with thorn at some points in time. Thorn became more popular entering Middle English while eth was more popular in Early English, though in very early English neither was present. Icelandic today uses both, with thorn more popular at the beginning of words, and eth used elsewhere.
Also, capital eth (Ð) should not be confused with “capital D with stroke” (Đ), which is used in Vietnamese to represent the /d/ sound (because the “D” character represents /j/—at least in southern Vietnamese dialects, it’s a different sound in northern Vietnamese). Other languages also use D with stroke for their own purposes. The lower case D with stroke is “đ”, different from eth.
