Greg Hurwitz wrote a response to a friend who asked him his opinion about that friend writing a book.  Greg posted his answer for the rest of us.

Strung together journal entries won’t work. They might make for a blog, but not a book. To write a book you have to write a book that is clearly a book and adheres to all the conventions and requirements of being a book. This is a shit-ton of work and will take drafts and time and sweat and blood until it’s either good enough to submit or you give up. As one of my writer buddies says: One of these will happen first.

Helpful to think about it, isn’t it?  You should take a look at the entire post here.