The Left Handed Booksellers of London sounds like an interesting premise, and thankfully one that gets pulled off reasonably well. One thing I will say right off is that it’s a lot more interesting if you know a little bit about British or Celtic folklore; I also think the 1980s setting doesn’t get enough attention and that’s a totally missed opportunity.
In a lot of ways the story is pretty standard: country child comes to the big city to pursue an education and find the parent they’ve never met; they get involved in something supernatural sort of by mistake, and get drawn into the strange and new to them world, but maybe it turns out they have a bigger connection to that must be discovered in order to save the realms from whatever the main threat is (this too needs discovering). The guardians of the supernatural human balance are called book-sellers (and often do this as a day job too), and they are divided into two types of specialization: right handed are the scholars with the lore, and left handed are the fighters with the battle skills when the bad things get too ambitious. Both deal with magic in some way.
A lot of the time, it feels like heroines in this position seem to either fall into the needs saving from their ignorance range (like Alice in the Nightjar duology) or they have a temperamental chip on their shoulders (kind of like Rupa in the Library of the Dead series); thankfully Susan happens to be a good mix of independent and feisty but also capable of listening and learning to the people she needs to listen to. Her main first link to the supernatural world is Merlin who is also a potentially disastrous stereotype but also avoids the traps. Merlin comes off as somewhat powerful and knowledgeable with some ambitions but also kind of superficial and queer; he’s male most of the time (early on at one point he tells Susan “we’re (Booksellers) a bit shape-shifty…I was born male but I have been pondering if I should change”), very into clothing, seems to be a bit of a drama queen, and doesn’t deny it when his sister points out his relationships seem to not last very long. Merlin’s humor and occasional touches of emotion keep him sort of real, and he and Susan actually do end up making a pretty good team; his sister Vivien acts as a good balance but not a third wheel.
I know the sequel is not out yet, but I’m pretty sure I’m going to pick up the Sinister Booksellers of Bath, both because I want to see what happens with and Merlin et al., and also because of the pun in the title (it’s takes some seriously nerdiness to see it, but it’s there). Maybe then the 1980s London or England at least might get more attention; Susan makes a brief mention of the time period she’s supposedly in but that’s it; having lived through a good chunk of the decade myself, I think the general aesthetic of the history might actually have some interesting parallels to/commentary with the folklore.