How does this sort of thing work legal wise? If it's a book using the likeness of pre-existing characters, are you allowed to sell it? I'm aware there may be loopholes regarding the fact that your versions aren't 100% likenesses of the original characters, but I honestly don't have a clue how this sort of thing is treated. I know that some companies can be very strict regarding fan art.
Care to share your wisdom, oh Great Master of hard work and ungodly skill? ^_^