My wife makes a decent living writing (though not nearly as much as I make not writing) because she has both writing ability and a scientific background, the combination of which is rare.
She has three books out under her own name (from Scholastic) and one just turned in, and, unfortunately, it has not made her rich. She experimented in taking royalties on one instead of a one time lump sum - she got less.
She makes her money writing and editing on-line encyclopedia entries, which pay reasonably well once you are in.