Where can I read more about the topic of glyph variations?
Glyph variations are discussed Chapter 7 of the Unicode Standard, in the vicinity of Figure 7.1. For character/glyph relations, see also UTR #17, Unicode Character Encoding Model, and a paper posted on the Apple developer site on the Character Glyph Model by John Jenkins. Glyph variations in mathematical context are discussed in UTR#25, Unicode Support for Mathematics. See also FAQ: Variation Sequences.