I'm more excited for this book now. I don't need another encyclopedia of monsters. The origins, dispositions, and lairs (LAIRS!) information interests me greatly, though, and since I like a cosmopolitan setting, the races would seem like a good fit. I'd get that book even if it just re-hashed the MM, so the bonus monsters are icing on the cake!
The conceit of this being Volothamp's guide is good "plausible deniability" territory, and lets WotC go wild with monster info while inoculating themselves against (most) cries of puriists. The Volo/Elminster interaction seems a bit twee, perhaps, but we shall see how that plays out!