Start with ubuntu manual or the official ubuntu docs or debian docs
If the manual is not enough then there a nice shell guide where they learn you ancient unix commands in the terminal
What is next ? maybe some programming languages books like python or sql books if you are inclined to learn something new.
The next level would be drifting through the Linux manpages , this is what i did in my journey
At the end is nirvana learning the kernel internals and the source code (it's English slang and simple c) and can be tweaked at will and anyone can send patches upstream
ps: never ask why flash is slow on Windows , Linux , Mac : adobe bug she hates us "She's always hungry. She always needs to feed. She must eat."