You might know log lines by another name. Elevator lines is my favorite name for them because it tells you what it is: a line you would use to describe your book if you were in an elevator with an agent or editor and had only until the next stop to talk to them. So now you know all you need to know, right?
By whatever name you call them, log lines are …difficult to write. But they are so worth it.
Reducing your manuscript to one sentence that sums up the whole book helps you define it. And thus, makes it easier to write or edit. That might be why you want to keep a log line in mind as you do those things.
As an example, let’s say you have a log line that says “Twinkle Twinkle is a S/F novel about a man who harnessed the power of the stars to make his interstellar ship run and found himself with a bigger problem than lack of power.” Nobody is saying this is a good line, just an example. But, right then and there, you know pretty much the word count, the genre, that it’s about a hero not a heroine. It doesn’t include any love interest or vampires, or any historical aspects since we don’t regularly engage in interstellar manned space travel yet.
An elevator line isn’t something you’ll just jot down and never mess with. But spending a little time on it can tell you what is important to you about the book. Then you can figure out how to make that the most important part of the book.