There aren't too many books on the very instant of the Big Bang itself (if there was one); there are some about Big Bang cosmology, which is about early universe and its subsequent expansion. Which were you looking for?
Well, the classic (but outdated) book is The First Three Minutes, which (as the title implies), concentrates on the first three minutes after the Big Bang. The very early universe is also treated in The Inflationary Universe, by Guth. I've heard some people like The Whole Shebang by Ferris, but I've never read it myself.
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