The first modern paperbacks published are from 1935 by Allen Lane under their new imprint - Penguin Books & the Bodley Head. The initial launch was 10 books, by 10 different authors, of which two appeal to collectors of mystery fiction. These two had green covers - the colour choice for mysteries - and were Dorothy Sayers's The Unpleasantness at the Bellona Club (book #5), and Agatha Christie's The Mysterious Affair at Styles (book #6). Finding any one of the original 10 books is a great challenge for collectors, and a reason to scour used bookstore shelves. However, for the collector and fan of Christie's works, finding book #6 is a great accomplishment.
There's a great article about these 10 books on the Penguin site - click here to read more.
Values: Prices will vary heavily based on condition. A poor copy may only be £10 / $15, which a near fine copy could easily bring £200 / $300, unless you get lucky. Good luck!