I am assuming you want linux to talk to the internet, rather than manage some GUI screen etc.
I would suggest a raspberry pi and python for embedded webserver is where I would start.
There are some older books available on embedded linux, but you had to spend cash on a dev board, which wasnt really justifiable for me as a student.
You can read a lot about the linux kernel, but if you want to get your hands dirty, you can muck about with a pc/vm for cheap.
I was looking at
FriendlyArm since they were cheap kits on aliexpress. I would be using something off the shelf, rather than from scratch.
Still I think a pi for $45 with an atmel/arm board to manage IO would be cheapest/easiest way to make something.