In fiction and non-fiction the most important part of your book will be the synopsis.  This is ironic, because the reader may not ever see it.  On the other hand, it may be the only part a potential agent or publisher reads.

We didn't write the synopsis for Broken Toys: China's Song until some time during the second draft and we were wanting to send it out to publishers.  Most publishers will want to see a synopsis and three sample chapters. Trying to distill an entire novel down into a one-page synopsis was difficult.  Brevity is not something I do well.

When we finally did manage to compress the plot points to a single page, I found myself thinking, "Should I have made this a children's book and figured out some way to get pictures?"  In retrospect, I think I was more able to fully flesh out the characters and bring more meaning by writing a full novel.  However, the process of writing the synopsis brought out several things to me, so I suggest that you consider writing the synopsis as soon as you have the basic plot and characters in place.

First, it's a good idea to put on your marketing hat before you spend a lot of time on something.  If you aren't going to be able to put together a decent synopsis, you're going to have a difficult time selling your work.  So, you may want to move on to something more marketable.

Second, you need to be able to distill your message down to a page. Really, you want to be able to say in a sentence or so what your book is about. This should give your work some focus. Also, if you meet a publisher or agent at a conference you can blurt out something like this:

"I'm writing a book about how God is in the business of fixing hurt and broken people.  I use broken toys and a kind toymaker as an allegory to show how much God cares for us."

Hopefully, that will elicit a "Tell me more" response.

Third, looking at the synopsis should give you a fair idea about the length of the finished product.  You should always try to tell your story in as few words as possible, without losing the depth of the characters or the meaning.  Of course saying that is much easier than doing that.  

JK Rowling's later books were famous for their high page count.  Ironically, one of the great things about Harry Potter and the Sorcerer's Stone is that it sets up the entire Wizarding world in Just over 300 pages.  She wastes very few words and almost every scene is important.

One of the reasons A Christmas Carol has had such staying power is it tells a very engaging story in so very few words.  After all, when's the last time you saw the Great Expectations Christmas Special?

Finally, reviewing your synopsis should tell you if you've left anything out.  This is especially important for non-fiction.

So, what has the process of crafting a synopsis taught you?

Once you have several ideas, how do you choose which idea to use?

If you can be sure which idea is going to be most marketable, then that may be one way to decide.  One of the advantages of the e-publishing market is that you can publish information for a particular niche very economically.  Try to spend some time researching your market to make sure your book will be similar enough to the other offerings while being different enough to be distinctive.  For instance, the romantic comedy where the couple doesn't actually end up together may be creative, but may not garner you many fans.

Another way to choose might be which idea has a message that you think is most important.  You may choose an idea with a message that you are passionate about over an idea that is funnier or more creative.  The trick is talking about your passion without getting "preachy".

As for Broken Toys: China's Song, I chose to write it over a notebook full of other ideas for one reason:  the characters would not leave me alone.  Any time I let my mind wander, it would wander into their world and I would learn a little more about them.  A cast of characters that are original, funny and creative, will always make a lesser idea better than a great idea with two-dimensional characters that the reader has a hard time caring about.