Tell me a story in five words or less. That was the first bit of advice I got on how to title a book. A little tidbit about me: I struggle to keep a query letter under 250 words and the back copy of a book under 200. Now you want me to do it in FIVE? FIVE? Are you nuts? No. It's sound advice, a great starting point.
It's not really how I do it, though.
Instead, I opt for what makes the novel...novel. Does it work? If by "work" we mean "drives sales," I have no idea. No reviewer has ever said, "I bought this because of the awesome title." Similarly, no one has said, "I skipped it because of the crappy title."
For my high fantasy series Fire Born, Blood Blessed, each book is named after a god/his eponymous nation. The titles are also the path our heroine takes on her journey. While each book is/was easy to name, those names do not tell a reader unfamiliar with the series anything about the book. I hope the made-up nation names at least hint that the book is fantasy. It certainly doesn't meet the SEO standard that Jeffe mentioned on Sunday (though SEO bears consideration in future naming efforts); however, the series name might.
As for my Urban Fantasy series The Immortal Spy, I went for the classic naming convention of adjective + noun. There will be seven books in that series, and the first four were easy to name because I already knew the rough plots. The titles reflect the spy for whom our protagonist takes up the mission that drives the plot. The titles also stay within a certain character limit (as in number of letters, not body count) so I don't dork up the layout of the cover.
The Burned Spy, The Plagued Spy, The Captured Spy, The Hanged Spy
I opted for straightforward naming with the UF and went weird for HF because I think the genres support them. I kept the titles short to be easily remembered and allow the cover art to hold greater presence. I used the series names to supply information the titles didn't and cover art to fill in where the words failed.