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While I can't give you an exact definition of this topic, I can give you my personal opinion on the differences. To me, the difference between the erotic books and erotica is mostly the plot . A story with erotic components can be based on romance actions a what have you where the story does not depend on the sexual content. Erotica, on the other hand, has stories based on sex and thus will have many more erotic scenes in it.
For example, A story about a secret agent working for the government infiltrates a terrorist cell and works to destroy them from with-in when he finds is one true love.
or
A story about a honeypot agents who infiltrates a terrorist cell using her sexual prowess to extract information from its members and uses it to destroy them from within.

Hopefully, this helps to answer your question.