I learned X86 ASM by sinking my teeth into the Intel 8085 manual, then lighting up LEDs on a hardware emulator and later on a 8085 simulator that me and my brother built.

What certainly helped was that I had did some digital design and instruction set architecture, etc.

Later on, I did some real-world assembly programming for the PIC microcontrollers and some inlined assembly in C, which I did not find daunting at all because of my previous experience.

I guess the best prerequisite for this material is having done some low-level C, the kind where you know about text/data sections and being comfortable with calling conventions, the run time and the linking process.