Linux has decimated Windows in the server market. It would be unthinkable for a new project to use Windows server, even Azure assumes you want to use Linux.
There are lots of industrial applications where open source has dominated the market. As the end user you might not see it, but almost all software and digital infrastructure you use has open source components.
Reading books. There are some great books on programming out there. I would strongly recommend a A Players Guide to C#. It’s structure, practice problems, and explainations of the basics were far better than any free guide to programming that I’ve seen online. There are a lot of other great books out there too.
It also doesn’t matter too much about a book being outdated when you are only studying the fundamentals.
A lot of the stuff you read online has never been fact checked or edited for clarity. Some of it is great, but most of it is not.