[XeTeX] "Book" font weight

[Nicolas Vaughan]

Hello,
I'm trying to use as my default roman face ITC Esprit Book. But when I use:

\setromanfont[Mapping=tex-text,%
  Ligatures=Common,%
  Numbers=OldStyle,%
  ]{ITC Esprit Std Book}

I get a missing-font error.

And if I remove the "Book" part, I get ITC Esprit Std Medium as my roman
face.

How can I distinguish apart the medium and book weights?

Thanks a lot!

Cheers!

Nicolas Vaughan
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