I am so conflicted about this book. On the one hand, it's funny, pays homage to golden age crime fiction, specifically, but not only that of Dorothy L. Sayers, and does it with both wit and wholehearted enthusiasm. It's an entertaining setup for a series to come, has likable characters, dozens of easter eggs for like-minded fans of the aforementioned golden age crime writers, is set against a publishing and library background which is jolly, and makes some interesting social points.
On the other hand, it gets some silly details wrong - it's set in 1935, tights are mentioned instead of stockings, a man wonders around the Cafe de Paris holding a Nebuchadnezzar of champagne in 1 fist - it's a 38-kilo dead weight, I've tried lifting them, it's not a one-handed job. More importantly, there are some weird continuity issues around the main characters and all of it should have been picked up by an editor. Harriet F. Townson is a pen name for Harriet Evans which makes it all the more surprising.
On the whole I really enjoyed it and think this is a series that's going to be worth following. I will never not appreciate a chapter heading of 'Miss Pym Supposes', it might be a while since I've read Dorothy L. Sayers but there were references I recognised instantly, and a very fitting almost direct quote from the punt scene in Gaudy Night (if you know, you know). The inclusion of people of colour, queer characters, and Jewish ones is done smartly and underlines both the diversity of pre-war London and the politics of the time. References to real-life characters such as Baba Metcalf and Brenda Dean Paul (I looked her up) are well used too. If you don't know or remember who they are and do look them up it adds depth to the reading and serves as a reminder of how much more colorful real life can be than fiction, but it's done with a light enough hand that there's no need to follow up if you're not inclined to.
My biggest issue was with Dora and her mysterious money. We meet her escaping from a thoroughly unpleasant fiance - initially, she's described as swiping all the cash she can find in the more or less abandoned for now family home, she's meant to have inherited money from her mother but doesn't know what's become of it, later there's talk of a bank account that she's using (an unlikely thing for a girl to have in 1935) then there are postal orders that she's spent on clothes, and finally, she's been saving every scrap of pocket money for her train fair. As the money discussions are generally accompanied by reflections on how financially precarious a woman's situation was in the 1930's especially those who had been bought up to be ladies it matters.
There's an aristocratic male character who seems destined to recur who's troubling too. There's a reason that Ngaio Marsh, Dorothy L. Sayers, and Margery Allingham made their aristocratic sleuths younger sons, so spoiler, the reveal about Ben (well sign posted throughout the book) undermines the believability of his character somewhat.
Overall if you love Sayers read it for the references and the jokes, enjoy it for what it is, and hope that the continuity issues are sorted out before the next book comes out. If these kind if details ruin a book for you though, it's going to be best avoided.