Although getting a book published and having success is a brilliant achievement, most new writers do not realise that the publisher will then want them to write book 2 pretty quickly and to a deadline.
This deadline is very important as the Publishers set up a whole team of people to proof read, edit, check for continuity, the printers are set up and the shops lined up for receiving the books. So if you miss the deadline you have put a lot of people in difficulty. Then you have to wait until all this can be set up again.
In addition monies for the book are split into parts such as, signing the book contract, sending in the draft manuscript, publication day.
Please don't think I am being pessimistic, it is just a number of would be authors imagine they will be sitting quietly, writing at their own speed and to their own timetable, which is definitely not the case.
However if you can overcome all this and be realistic about the work involved, it can be a very satisfying vocation (not many authors retire).