For FASM related information I recommend this course:
The instructor is also preparing a course on reverse engineering x64 on linux.
The Intel manual volume one is giving pretty decent introduction into processor details. I grabbed the whole 5 volumes (4800 pages :-O )
A more interactive site on x86 opcodes, registers, flags, instruction sets, etc.
x86 instruction set reference - another resource. Haven't used this as much though and just do CTRL+F on the Intel phone book :)
Difference between kernel space and user space in Linux. When I needed to search for how syscalls work in Linux
Found this quite useful when searching about MOVZSX and how stuff is stored in the registers and memory.
On YouTube I found 2 resourceful that are quite good to get your feet wet.