There are many reasons why you need a good book title. I will name a few in this chapter.

It's the essence of the story. Something that has to do with the plot...

It grabs the attention of readers. Something like "Gods calling." Sounds good. I normally get ideas from the site fantasynamegenerator.com and combine some together, or take some apart to make it fit what I'm writing about...

Make sure that its memorable. Ya don't want your readers to forget the title after the first chapter of your book, Right?

Title length has to do with this too. Some titles are better short, while some are better long. It depends on what your writing about. My book "Crimson River," is a somewhat memorable title, it kind of grabs the reader's attention. The book itself is somewhat okay.

You can base it off of a character's name, or nickname. Evoke emotion. Use imagery. Dialogue from the story itself. Create mystery. Use wordplay. Use humor. A metaphor. A characteristic of a character. There is a lot more possibilities. Crimson River is based off of the main character, River, who later gets wings that are a Crimson and orange color. Hence the name, "Crimson River."

Get input from others, ask for advice.

Don't expect to find the perfect title within hours though... Sometimes you need to take a break from titles sand return to the process later. You can probably find titles in music, or nature. Or just close your eyes and enter your imagination for a title. I'm sure you have quite the imagination if you are writing a book.