book recommendation for firmware engineers in C like Effective C++?

I'm leading a firmware (embedded software) team and I found that the members had very little knowledge about good coding styles. The problem is that I can't just say "your code sucks" and force them to change, I need to find some common rules in the industry and use them as the 'code of conduct' for the source code.

I have some experience of c++ and during my learning phase I felt the books of Effective C++ and More Effective C++ were very helpful: the rules are specific and practical. But our firmware engineers use C as the language, they don't even know what exactly a class is.

So I'd like to see if there are any books explaining some practical rules of firmware development in C language, just like Effective C++?

Thanks