The Lost Apothecary is absolutely "fine." I think the concept is great - three women across two timelines, wholly different intentions, with their stories overlapping - fascinating! Administering poisons to loathsome men? Wicked! A brilliant idea for a novel... in theory.
Unfortunately the execution lets it down. The storyline is entirely unrealistic throughout (the main [modern] character just so happens to find something, just so happens to chat with someone in particular, all her investigation just to happens to lead her to get the bullseye every single time...) which, although based in a fictitious world, comes off as entirely unbelievable. For me, it took away from the story, because I'd find myself almost rolling my eyes and/or thinking "yeah, right..." over and over. The main-character-is-so-special trope is suitable for younger audiences, and call it my aged cynicism, but it was all just a little too rosy-posy for me. It feels like there isn't a great deal of critical thought behind the decision making of the characters or the writing by the author, which I believe would have added some depth to the story. It would have been a bit more thrilling if something hadn't gone quite right and the main character had to work a bit harder, rather than have all the answers land in her lap, in my opinion.
Sarah cannot be blamed for being American. However, her copy editors can be blamed for not picking up on where she uses American dialect for her British characters, and adopting an entirely American perspective of London. Two of the British characters use phrases like "sneakers" and "yes ma'am" and - things rarely if not never said in the UK, and she immediately befriends a British woman at work who somehow over the course of a few days, wants to have cocktails and plan weekend trips? Many of the relationships were unfortunately not believable in the slightest, and further tainted by random American slang.
Overall, I think this is a great book for 16 year olds(ish). The concept is fun, the writing is fine in and of itself, but somehow the execution just doesn't hold up as well as it could have. I did, however, finish it in two sittings, so it is a page-turner. Perhaps a good book on the beach if you don't want to be challenged in particular.
It's no Sarah Waters - but is like a distant, much younger cousin.
Concept: 5/5
Execution: 2.5/5
Readability: 4/5
Recommend?: Sure, for YA readers.
Overall: Generous round-up to 3*.