The only other book by Hans Fallada that I've read is Alone in Berlin, written just before his death in 1947 and based on the true story of a couple who were executed for distributing anti-Nazi material during the war. His own life sounds unremittingly grim.
Little Man, What Now? is an earlier work from 1932, written just before the Nazis came to power. A best-seller, it was turned into a Hollywood film that I can't track down.
Sonny and his Lämmchen - his lambkin - are a young couple struggling through the economic crisis that saw the rise of Hitler.  (Forty-two per cent of German workers were unemployed.) Sonny is a little white-collar worker, a department-store shop-walker, clinging to respectability but his life is even more precarious than an industrial worker who has some solidarity with his comrades. Sonny gets his girl pregnant - he marries her - they're hard up but deliriously happy and so much in love that it doesn't seem to matter that she's a disastrously inept cook and they can't manage their money. The sums are never going to add up and when he loses one job, and then another - he lives in dread of not achieving his sales targets - the little family are on a downward spiral ...
As I read, I couldn't help thinking of Greenery Street, a 1925 novel about another young couple embarking on married life in Chelsea. But how different their worries are, unable to manage their servants or live within an income of £1000 a year. It's frothy, romantic and silly ... and how bored I got with Felicity, the silly wife who is so completely different from stoical Lämmchen.  But PG Wodehouse's comment holds good for both novels: 'It's the only possible way of writing a book, to take an ordinary couple and just tell the reader all about them.'