You should probably join the author career if your sim is going to be writing books anyway.
The daily career task for the author career is to write books, you get a steady salary in addition to your book royalties, you get to meet co-workers for friends, and you get career rewards like the book that gives an inspired aura and the computer that looks like an old style word processor.
The only reason not to join the career is if you want your sim to be in control of their own work hours, e.g. to look after kids or work on other skills or hobbies.
