Zadie (originally chosen by the writer at 14) is a variant of Sadie, itself from the Hebrew Sarah ("princess"). Professor of Fiction at NYU.
Zadie does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Zadie reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.