Titles seem to take as much time and thought as re-writes. Getting just the right one that will attract new readers, not put off familiar readers, and sum up the novel without giving too much away, is a daunting task.
For the thriller, I've tried Crimes of the Early Morning (too artsy), Killing People Is Wrong (too whimsical), Murder Casts a Long Shadow (too melodramatic). I think I've settled on The Gibraltar Coordinates (intriguingly enigmatic, I hope).
For the drawing room mystery, I think I've settled on The Dead Scholar. I've tried a lot of others, including Dead Reckoning (my publisher thinks it's too common, although it has only been used once, in 2005, and once in 1978), and, my favourite, A Goodly Huge Cabinet (an allusion to Bacon's notion that all informed people should have a "cabinet of curiosities" to keep the souvenirs of their lives for safekeeping—too esoteric). And others, as they say, too numerous to mention.
Now, I need to decide, which comes next, after the psychological emphasis of Still Waters and the gothic play in Grave Doubts??