Tonal Sandhi

For deriving a tonal conlanguage from a non-tonal stock proto-language
moved into a tonal sprachbund.

Can anyone tell us anything about tonal sandhi?  That is, about typical
patterns of how tones on "morphemes" or syllables, as they exist when
standing alone, get modified when they are combined into a word?

Can anyone tell us anything about typical patterns of diachronic evolution
of tones? and of tonal sandhi?

Can anyone recommend any good resources to consult?

Thanks,
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eldin