Agatha Christie is much better with full length novels. That way she can properly build suspense and drama. When the stories are only 20-30 pages, the crime or situation has to be quickly setup and resolved without much investment in the characters.
With that being said, the stories are good, with clever endings, but they are not as good as some of her full length novels such as And Then There Were None and Murder on the Orient Express.
Eleven more quickies in this one written between 1924 and 1948 (The Witness for the Prosecution, The Red Signal, The Fourth Man, S.O.S., Where Theres a Will, The mystery of the Blue Jar, Sing a Song of Sixpence, The Mystery of the Spanish Shawl, Philomel Cottage, Accident, and The Second Gong). For a refreshing change we are not bothered with Poirot, Miss Marple, or even Parker Pyne. Whew! Did I speak too soon? The last story is a Poirot. Anyway some are interesting with an unexpected finale; others are downright boring. Typical easy reading for an afternoon or evening.