Any fool can write a book. I know, becuase I am as much a fool as the next person, and I have written one. So writing a book is not that hard.
What is hard is writing a book that someone else will publish. So what does it take to get someone to publish your book? Well you have to convince a publisher that what you have written is somethign they should want to publish. Publishing houses ususally restrict themselves to certain genres of books, having already determined they want to publish for a particular audience. So you have to convicne the publisher that what you have writtn will reach thier audience. You also have to convince the publisher it will sell.
The way you do that is through a successful book proposal. You have to write that too. Writing the proposal is infinitely harder than writing the book itself, nevermind that it is proabbly just a fraction the length of the book itself.
Yes, I am in the process of writing a proposal. And I am struggling with it. Yuck.