I don’t have any beginner-specific material for optimizing Assembly.

But after skimming a few of Agner’s manuals, I think “beginners” means “new to programming” or “new to computer science”. Not the optimization aspect.

If your level of programming knowledge is “I learned to write Hello World in Python last week!” then Agner’s material may as well be hieroglyphics. But if you’ve got some C or C++ under your belt, have written a little assembly and can articulate the difference between a register and the stack then you should be able to understand Agner’s manuals.