Is KDP Select good or not given the 90 Day exclusive period?

Since, you're considering enrolling your ebook in the KDP Select program for the 90 day exclusive period, you'll want to think about the following...

Set your calender to ensure you don't auto renew the program. Price your book a bit higher than you normally would while the book is in the KDP Select Program. For instance, if your book is selling at $2.99, you'll want to increase the price to $4.99 that way there's a perception of increased value to the reader who may select your book from the program. Quickly work on creating  a second installment of your book ... fiction or non-fiction. People who read your first book will most likely want another from you and this is where the real goldmine is in the process ie future sells of other books.

Time the release of your second book so that it's published not much later than after the 90 day KDP Select Program has ended on your first book.

Remember to use the FREE days that Amazon offers to promote your book as well. Many of us have found the greatest level of success is in terms of downloads on Fri, Sat and Sun. These days become a little skewed when you factor in the global market ie Fri. may be Thur. or Sat in another part of the world. To keep things easy focus on your home country.

Actively market and promote your book while it's in the KDP Select program along with the free days to maximize exposure.

Create and publish high quality ebooks that people actually want to buy, for details, you'll want to read the 4 Steps to Publishing your Ebook!