Respectively, these books are numbers 8 & 9, with Peril being published in 1932, followed close at hand by Lord Edgware, which was published in 1934.
Also in 1934, she published one of her best known novels, The Murder on the Orient Express.
I thoroughly enjoyed both books, and it is difficult to believe that the nimble, clever author of these books also wrote the hackneyed, plodding The Big Four. Of the two, I thought that Peril at End House was more clever and more convincing - Lord Edgware Dies relied a bit too much on coincidence for my taste.
The two books are a matched pair, as well, because they feature a pair of truly evil antagonists whose self-centeredness is only matched by their avarice. One thing about Agatha - she is unafraid to write characters who demonstrate that evil is an equal opportunity characteristic. No passive female innocence here - her lady villains know what they want, and they aren't afraid to use violence to get it.
In addition, for a change, the villain in Lord Edgware Dies is actually brought to justice. I've been meaning to do a post for a while about Dame Agatha's utter lack of interest in any sort of criminal justice consequences for her bad guys (and gals). My thesis gains traction with nearly every book - Lord Edgware Dies is the rare Christie where the murderer is tried, found guilty, and sentenced under the criminal justice system.