The most important information that you require is that produced by the manufacturer of the chip. We are extremely fortunate that both AMD and Intel make the Programmer's Manuals, and many other useful documents, available at no charge on their web sites.
Whichever make of processor you have I would strongly recommend looking at both sets of documents. The differences between the actual chips are minimal, but the manuals provide interestingly different slants on them.
From here on I will refer exclusively to the AMD manuals. A reference to AMD2 (for example) means AMD64 Architecture Programmer's Manual Volume 2: System Programming.
Some books that I found extremely useful are:
These two aren't OS-related but will probably be of interest to anyone who appreciates the intellectual challenge of writing an operating system:
There are a large number of websites that provide useful information, and I find new ones every day. The amount of information available for free nowadays is just amazing (particularly to those of us who grew up 50 years ago). Three sites that I would particularly recommend are:
Those are just three of my favourites. Google a bit and you'll find your own favourites.