The best way to tell how good your self publishing book idea is, is to look at how easy it is to market. If it is difficult to sell then it simply isn't a good idea.

The best self publishing book idea will be interesting and a breeze to market mainly because it will tend to focus on solving certain pressing problems in the target market that the self publishing entrepreneur has identified and chosen to focus on. If for example a blog is being used to market the book, it will be fairly simple to identify the keywords and keyword phrases that will tend to best attract traffic from leading search engines. The hits attracted to the blog in this way will be exactly the target market niche that was being aimed at for the book and will therefore be very easy to market and sell to.

Actually the narrower the niche your best self publishing book idea is targeting, the better and more successful the book is bound to be. One of the mistakes first time self publishers make is to try to be everything to everybody. It never works in any business, and there is no reason to expect it to work when you are self publishing a book and therefore it is in your best interests and a good idea to avoid being general at all costs.