I don't have any link in mind (I think google + linux + embedded would do a better job than me)... But I have made from scratch some embedded systems based on linux, it's really easy if:
-you understand the linux organization (what's where),
-you know how its booting process works (rc.d and friends),
-you know how to boot on a ramdisk, and use it as / (distro generally boot on ramdisk, but then chroot to / on hard drive).
-if you have to deal deeply with the kernel (not just compiling it): "http://lwn.net/Kernel/LDD3/"
-Using busybox to have a minimal shell and set of commands for your system.
After all, an "embedded linux" it's just an simplified linux...