PLEASE LET'S NOT HAVE ANY SPOILERS AS IT WILL RUIN THE SERIES FOR THOSE WHO HAVEN'T READ THE BOOK!
Thank you.
The three best Agatha Christies (in my view) are this one, Murder on the Orient Express and The Murder of Roger Ackroyd. Every one a cracker. Having said that, she scarcely put a foot wrong in anything she wrote from the mid-20s to the early 50s. The quality is a bit more variable in the last few years of her career.
The 1930s were anything but twee! In the early 30s the Great Depression caused terrible hardship in the USA and the UK. Stalin was in power in the USSR and millions of people were sent to labour camps or executed as part of his great purge in the 30s. Fascism was on the rise all over Europe, Jews and leftwingers were fleeing Germany and central Europe, the Spanish Civil War was like a warm up for WW2 and by the late 30s it was clear there was going to be another world war.