Ha'penny is a loose sequel to Farthing; it takes place a few weeks later, with Carmichael investigating another crime, and with a different young woman providing the first-person voice. Like Farthing, it's again a woman of aristocratic family who has great sympathy for the people getting the short end of the stick. This time, it's Viola Larkin, an actress and thinly-disguised Mitford sister; she has Diana's place in the age line-up, but something like Nancy's politics. (Though nothing like Nancy's wit and insight, or the plot would be very different.)
The crime this time is terrorism: a bomb has exploded in a quiet London home, and Carmichael must find out if it was deliberate or an accident...and if any plotters are still active and planning bombs for other locations.
At the same time, Viola has gotten the part of a lifetime: she is to play Hamlet in a major gender-reversed production. And her premiere performance as the conflicted Dane will be attended by Chancellor Hitler and England's new Prime Minister Normanby, who is himself in the process of shoving through "reforms" to consolidate power. And one bomb in the right place on the night could save two nations...or throw them into bloody repression for a whole generation.
Of course the two plots intersect, and you can probably work out the general outlines of that intersection, if you want to -- but you'd be much better served by just reading the book.
Ha'penny is not quite as successful as Farthing, but it's a somewhat more honest book. In Farthing, the government plotters called their enemies "terrorists" as a pretext to round them up or dispose of them. But in Ha'penny, we see that there really are anti-government terrorists, though they're not the "Jews" and "communists" that Normanby claims to be rooting out. And we might even sympathize with them, as we see them planning their murders.
I do have to admit that Walton can pound on her thematic materials (Hamlet and the "Larkin" sisters, plus a lot of people talking about security and fascism, racism and fear) with too heavy a hammer. She clearly intends these novels to mirror our own times, which is not necessarily the best way to write fiction. Luckily, she doesn't make the parallels explicit; the fascism in these two novels is a very homegrown English sort, built on the class system and the old-boy network -- exactly the sort of society that, in our world, WWII shattered into pieces forever.
Walton is a natural at writing detective fiction; the story flows out wonderfully, dragging the reader along in its wake. I had to put this book down for a while, but I hated to do so -- Viola's actions deeply annoyed me, but they were still exactly the way I'd expect a woman like her to act. Ha'penny is something of an anti-mystery novel, as Farthing was; they're both books that don't renormalize the world at the end, that don't set the world back in order. This is a world that cannot be set back in order by the actions of a few people acting in secret. But I hope it is still a world that could be set into order, that isn't irreparably broken.
Ha'penny doesn't have the white-hot fury behind it that Farthing did; it's a colder, calmer, more collected book. And so it might not be as immediately impressive, but, in its portraits of ordinary people living with endless dread, it achieves a chilling power all its own. It's a fine work of alternate history, a tightly-plotted mystery, and a nasty look at a world we can be happy we didn't get.