A young woman confesses that she thinks she may have committed a murder, then vanishes, leaving Christie’s legendary sleuth Poirot desperate to find her and to figure out just exactly how one could not know whether they were a killer or not.
This was a little slower than some of Christie’s other tales, but is good nonetheless. This slow burner presents a complex mystery where reality is subjective and not everything is as it seems.
From Goodreads:
In this breathtaking Agatha Christie mystery, the Third Girl sharing a London flat with two others announces to Hercule Poirot that she’s a murderer and then disappears. The masterful investigator must figure out whether the missing girl is a criminal, a victim, or merely insane.
Three young women share a London flat. The first is a coolly efficient secretary. The second is an artist. The third interrupts Hercule Poirot’s breakfast confessing that she is a murderer—and then promptly disappears.
Slowly, Poirot learns of the rumors surrounding the mysterious third girl, her family, and her disappearance. Yet hard evidence is needed before the great detective can pronounce her guilty, innocent, or insane.…
If I had to name my first ‘literary hero’, the first author whose books I read that made me jealous, that made me think to myself, “This. Is. It. This is what I need to do. I need to write stories, mysteries, that makes people feel like this book has made me feel,” only one name come to mind. The author is Agatha Christie, and the book was 
Shannon Hollinger 