Reading materials
x86 ASM
PC Assembly Language. Paul A. Carter.
Brennan's Guide to Inline Assembly.
ASM/Asm/Assembler opcode reference.
Calling conventions
X86 Calling Conventions (Wikipedia). General explanation of various calling conventions.
Calling conventions (DJGPP page).
x86-64 ASM
System V ABI(includes calling conventions among other details).
Windows x86-64 calling conventions.
Intel Manuals
Intel 64 and IA-32 Architectures Software Developer Manuals. This is the up-to-date manual for the most recent Intel CPUs (it's long).
For the instruction set you need the following parts (Volumes 2A, 2B):
Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2A: Instruction Set Reference, A-L.
Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2B: Instruction Set Reference, M-Z.
Intel 80386 Programmer's Reference Manual. An older i386 manual, which is much shorter and easier to read.