OSDev.org

I learned 8086 from a book, "80x86 Assembly"
It was easy to understand.

I learned 386 from Internet also

Before reading these old manuals, is there any rigorous required prior knowledge? I mean, I know about basic hexadecimal and that. But, what else, or is this how I begin?

You should probably have some idea about what "a byte", "a word" and "signal level" are, pretty much that's all.

Since these days you have google you can look up anything you don't know as you read it.