Since I have so many paper books to read, I am trying to hold off on buying books for my Kindle. There are hundreds of free pre-1923 books available. (These books' copyrights have expired.)
Agatha Christie's first two novels are pre-1923. I consider myself an Agatha Christie fan and hadn't read The Secret Adversary, so what a deal. The Secret Adversary is the first Tommy and Tuppence book. Christie wrote several Tommy and Tuppence books, but there aren't near as many as Poirot or Miss Marple stories.
I'm glad it wasn't the first Agatha Christie I'd read. I don't know that it was good enough to make me keep reading her novels if it had been the first. Give me a Poirot book instead!