I recently read a good article by Guy Kawasaki on how to decide whether should you write a book. Having himself authored 12 books, he articulated some of the reasons for writing a book and reasons for not writing a book, quite well.
There are four good reasons for writing a book:
(a) you have accumulated knowledge that merits sharing;
(b) you want to further an idea that benefits mankind;
(c) you want to entertain, amuse, or inspire people; and
(d) you want to conquer the intellectual challenge for writing a book.
Some of the bad reasons, and quite common ones, to write a book include:
(a) you believe it’s a means to an end where the end is positioning you as a thought leader, making a lot of money on royalty, or fostering your consulting and speaking career.
Still wondering why your previous book did not do as good as you expected, you might have just read the right reason for it.