A burning desire to get the book written. Also, the ability to be able to write well. The burning desire will keep you at it until it is written. The ability to write well will ensure that you at least get it printed and give a copy of it to your friends.
And, of course, my interpretation of your words "a good book" means that it will have real substance that comes not only from your own heart but will reach both the hearts and minds of your readers.