Ed is a lecturer of Computer Science in the College of Engineerin in University of Nevada, LA. It is a interesting source to learn more about low level languages in a free way, it counts with three books: Programming using Fortran, MIPS Assembly programming and x84_64 Assembly Programming. Ed Jorgensen books
OpenSecurityTraining2 has courses about Architecture1001-x86-64 Assembly, Architecture2001-x86-64 OS internals, Debuggers, Reversing Egineering, Malware Analysis and many other resources to learn. OpenSecurityTraining2