My personal opinion

Learning C first and then assembly for embedded applications is just like learning the driving in reverse direction.

You should have some know how of assembly language before you start programming micros in C.

8051 Microcontrollers from I. Scott McKenzy is a very nice book and has some good information on modular programming.

Also "Microprocessors Programming and Interfacing" from Douglas V. Hall although it focuses on 8086 family, but a really nice book.

May be the names of books I mentioned are not exactly same as they appear on respective title pages, but I am sure about authors.

Good Luck
IA