You can do a lot with QEMU. https://bootlin.com/ has a lot of great, free training material.
https://bootlin.com/doc/training/embedded-linux-qemu/embedde...
is one of my favorites.
Learning to cross-compile, do embedded debugging, the process of booting an embedded system (which varies depending on the answers to the above questions), learning how to read a technical reference for the processor you’re using as well as for peripherals you’re likely to interact with - SPI, i2c, UART, maybe PCIe, are all handy skills. Learn a bit about JTAG, hardware, reading schematics, etc. Even being able solder is helpful.
There may be books (I had a great embedded Linux book when I started) but there are lots of online materials too. Check out https://platformio.org/
There are fun embedded boards and projects for microcontrollers too - micropython on an rpi pico, tinygo, eLua, etc.
The book is available as Open Access PDF at https://link.springer.com/book/10.1007/978-3-030-60910-8