Don't go there. There's really not a lot of work. But if you must, low level is more a calling than a learned skill. You probably shouldn't be learning assembler. First, compilers are really quite good. Yes, it's possible to beat them (I do) but generally not by much. And not by much ain't gonna put bacon on the table. You can probably get what you need from gcc inline asm() calls. Take a look at the linux sources an…
Also: to say its a calling is accurate. I love what I do because what I create is physical and interacts with the physical world. I can hold my projects in my hands and say "I made this". I've done web and desktop stuff and can say that this work is significantly more fulfilling to me.