Plenty of great sources exist for information on firmware design. Here are some of the best.
Magazines (online and otherwise)
Embedded.com--You're reading it now. It's the most important publication dedicated to embedded systems.
EDN--Yes, this is a hardware publication, but hardware is never far from the software.
EE Times--EE Times is the source of information about what technologies are coming, where the jobs are, and what companies are up to.
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