Compartmentalize.

[harrisi]

No description provided.

[harrisi]

I'm not sure what this is referencing.

[ghost]

If it makes no sense you should probably not add it. This is referring to the lines immediately below it,

for (auto &c : initial_glyphs)
    book[string(1, c)] = glyph(face, (FT_ULong)c);

Suggesting that a bare map may not be the best way to store the glyph cache, but it's the best I can come up with without just adding a bunch of boilerplate that wraps it in a class.

A useful title might include something related to UTF-8 normalization which is necessary at some point (e.g. turn multipoint sequences into their condensed forms where appropriate, eliminating combining characters). See here under "Combining Characters." Or this could just be closed and a separate issue opened up for that one - it should be in a comment somewhere.