I'm not a professional embedded engineer, but I do hack around it. Some books I collected:

- Applied Embedded Electronics: Design Essentials for Robust Systems by J. Twomey. It goes over the whole process making a device and what knowledge would be required for each. Making Embedded Systems, 2nd Edition by E. White is a nice complement.

- Embedded System Interfacing by M. Wolf describes the signals and protocols behind everything. It's not necessary as a book, but can help understand the datasheets and standards

- But you want to start with something like Computer Architecture by C. Fox or Write Great Code - Volume 1 - Understanding the Machine, 2nd Edition by R. Hyde. There are better textbooks out there, but those are nice introductions to the binary world.

The gist is that you don't have a lot of memory and CPU power (If you do, adapting Linux is a more practical option unless it's not suited for another reason). So all the nice abstractions provided by an OS is a no go, so you need to take care of the hardware and those are really finicky,