I was worried that a book from 1998 would be too outdated

K&R 2nd Edition is from 1988 and still the recommended introductory C book. ;)

AFAIK (and I know nearly nothing about Windows programming), the standard way for writing user-space applications on Windows changed a few times. In the early days, it was WinAPI, then came MFC, then came .NET and I think nowadays they have this UWP stuff for Windows 10. So, WinAPI apparently got replaced several times already, but Windows versions are generally backwards-compatible so everything you write in WinAPI will probably work.

If you want to use C, you'll probably have to use WinAPI though, so go with that book.

Personal opinion: Fuck Windows, except for AAA gaming and maybe Windows-only software you absolutely have to use, there's no reason to use that closed-source, inelegant, bloated, spying piece of crap. By choosing to write in WinAPI, Unix users would have to use Wine to use your application. I'd pick GTK and if that turns out to be too cumbersome, write your application logic in C and the GUI code in C++ (or Python, or Java, ...).