It's £33 for 3 - can't remember price per book.
You can find more information about the books in their catalogue by looking at the Persephone website.
A lot of their books are originally written/published in the early-mid 20th century and there are a few about each world war, including some fiction and non fiction about WWII and about Nazism before the war (the founder of Persephone Books is herself the daughter of a German Jewish refugee couple who settled in London before she and her older sister were born). There are also several novels about suffragettes and an assortment of non fiction.
My personal favourites include
Barbara Euphan Todd, Miss Ranskill Comes Home - Miss Ranskill was marooned on a desert island for a few years, and has made it home only to discover it's wartime (WWII) and there are all kinds of issues of rationing. Some serious issues but a lot of humour in the handling of the way the war changed Britain.
Noel Streatfeild, Saplings - Streatfeild initially wrote for adults though most of these books have only just been reissued through small publishers (this is the only one from Persephone but many of the others are available as ebooks, several for under £3 at the moment). This is a rather sad story of a very dysfunctional middle class family but it's a good read, though I did buy a charity shop copy for my mum and then worry about giving it to her because it's so sad.
D L Stevenson, Miss Buncle's Book - lovely social comedy about a woman who writes a novel satirising the people around her in her village.
Rose Allatini, Despised and Rejected - Banned after publication in 1918, during the last year of WW1, for its portrait of conscientious objectors during war time (and lesbianism). I find it really interesting to read some of the wartime novels about wartime, as well as historical novels and those written with hindsight a few years later.