Aww, too bad you didn't wait a few years, since the early ones are about to go out of copyright. The first one already is
I'll tell you guys if the Marple ones are any good this summer, since I have them but haven't had time to look at them.
The Secret Adversary
is free too! Here's my thoughts on it from this thread:
"Fun in the Christie sense. The new protagonists Tommy and Tuppence are a fresh breath of air away from Poirot and Marple. They're just a pair of dimwits way in over their heads. The romantic subplots were dumb, though. The ending was a surprise. I love how she used cliches to both lessen the work (unintentionally) and make it better (intentional)."
No idea why I mentioned Marple, I haven't read any of her stories yet. Kroms you should pay attention to what you type.