My opinion - you get what you pay for.
The authors of the books need to make money, so they write books on what they know.
If free training is what you are looking for, there is always youtube or searches for blog posts or blogs/stairways on SSC.
Now, free isn't always bad either. But I find that with a lot of free training (videos, books, blogs, etc.), you do get what you pay for. There are some good books out there for free, but sometimes they focus on old versions of SQL Server or things that are not as much a best practice anymore due to advances in computer hardware.
Have you checked out the "Books" section on here? There are multiple pages of books and there may be one that has what you are looking for.