[chc]

The book says Apple opted to evolve its existing operating system, which was OS 9. This strongly implies that NeXT's contribution was negligible, but even if you don't see it that way, there is just no reasonable way you can read it to make it true. Mac OS X was an evolution of NeXTSTEP, full stop. Even if OS X contained twice as much OS 9 code as it appeared to, it would still basically be NeXTSTEP.

To wit: Foundation, AppKit, the BSD underpinnings, Mail, TextEdit, Preview, the Dock and countless other components — even that silly beachball wait cursor! — were refugees from NeXT. Even Finder, which borrowed its name and some of its appearance from OS 9's Finder, was essentially a Carbon rewrite of NeXT's Workspace Manager.