Don't Write for Money. Write Because You Have a Story to Tell.
I see an upsetting attitude toward writing, and that attitude is "Don't write the 2nd book in the series until the publisher accepts the 1st book and it sells enough to pay back the advance. If it doesn’t sell, the 2nd book is just a waste and useless so don't even start it. Better yet, make the first book a standalone."
To that I say NO! NO! And *expletive* NO!