Digital dropped the ball in the late 80s with regards to management of Cutler and his team, canceling his PRISM project and leaving him and his team disgruntled.
Elsewhere in Seattle, a chap named Bill Gates was flush with billions of cash and knew that the shelf life of DOS was limited; if Microsoft were to succeed, they needed a new, robust, reliable and high-performance OS that they could "bet the company on".
Gates got word that Cutler was disgruntled at Digital, and a mutual party set up a meeting. Cutler was dismissive of Microsoft's technology stack at the time (DOS and some office apps) -- he was a hardcore OS engineer, and DOS was a toy.
Gates persisted, ensuring Cutler that he would have the opportunity to build the next generation of OS from the ground up and essentially unlimited resources at his disposal to do it. Cutler eventually agreed, and the NT kernel project was born.
Reading the book and learning the story behind NT's development, it's just amazing that such a good OS came out of that process - they released years after their initial projections and were rushed the whole time. But of course the really good parts of NT - the kernel, the object manager, the pager, async IO, the threading model - were things Cutler and his cohorts had been working on for years, first with VMS, then with PRISM, and then finally in NT. They had YEARS to ruminate about those things before they ever arrived at Microsoft.
The bits of NT that aren't so well-regarded - the registry, NTFS, the graphical shell, csrss.exe and the 'microkernel' design - were completely new and developed in much less time and with less practical experience behind them than they really deserved.
I sent a tweet to the author saying I was really enjoying the book when I was about half way through it and he actually e-mailed me to say thanks. How nice is that!
Arguably, Linus' greatest work was Git, not Linux. Linux is, architecturally, a piece of shit! Actually, wait, so is Git. Mercurial does everything Git does and does it far better and more elegantly. So yeah, wait... one wonders where Linus gets all his fanatics from!