There are professional and hobby authors, and in literature you often can not draw a clear line of distinction between them. A simple definition is that if you make your living with writing, you're a professional author. Unfortunately, this excludes most commercially published fiction authors, most of whom must have a "day job" to support themselves. Another definition is that if you have been published in book form by a commercial publisher (not a vanity press, nor a self-publication), then you're a professional author.
A co-author is a person who shares authorship of a work with one or more other people. It is often not easy to tell which of the identified co-authors contributed particular portions of a work, or how the various co-authors interacted and modified each other's contributions. In absence of other information it is usual to assume that each co-author contributed equally to a work.
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