Start with a computer architecture introduction. The McGraw Hill Computer Science series book "Computer Architecture" did a good job of creating a fictional processor and then designing the machine code for it. "Assembly" is just a way to represent machine code in text files.
That way you will learn what it is the computer is trying to do, and how constraints on how it is built change that.
Then I'd suggest some cheap 8 bit Microprocessors like the AVR series and the PIC series from Atmel and Microchip respectively, (the AVR has solid C support so its probably a better single choice, but the PIC has weirdness associated with architecture constraints which is good to understand as well).
Once you are a pro writing AVR assembly code, then grab a copy of x86 assembly and a description of the Pentium architecture. To do it proper justice start with an 8086 assembly book, then a 286 assembly book, then a 386 one, and finally a Pentium one. That will let you see how the architecture evolved to deal with the availability of transistors.
That way you will learn what it is the computer is trying to do, and how constraints on how it is built change that.
Then I'd suggest some cheap 8 bit Microprocessors like the AVR series and the PIC series from Atmel and Microchip respectively, (the AVR has solid C support so its probably a better single choice, but the PIC has weirdness associated with architecture constraints which is good to understand as well).
Once you are a pro writing AVR assembly code, then grab a copy of x86 assembly and a description of the Pentium architecture. To do it proper justice start with an 8086 assembly book, then a 286 assembly book, then a 386 one, and finally a Pentium one. That will let you see how the architecture evolved to deal with the availability of transistors.