I think the highest compliment I can give this book is to say that I am so, so excited for the next book! My positive experience with this book is such that I want to see it on the shelves in bookstores next to titles from the NYT list; it certainly 'deserves' it more than... certain other books.
Here's how I described this book to a friend I'm trying to persuade to read it:
-This is a character-driven story focused on one woman's experience with PTSD, the effects of which are demonstrated and amplified by supernatural occurances
-The characters are incredibly 'fleshed out' and have refreshingly realistic reactions to supernatural situations, applying reasonable explanations rather than the usual immediate acceptance or total denial. They are all realistically flawed, but the portrayal of those flaws isn't done so in a way that makes me dislike the characters or not want to spend time with them
-Sexism, especially sexism displayed in how women are expected to be caregivers for men, is an important theme of a B-plot sort, with what I thought was a very satisfying 'resolution'
-The portrayals of PTSD and trauma are heavy, but don't bog down the plot or make the book unreadable
-There are some very funny lines for those of us who appreciate some dry British humor
-The MCs lesbian-ness is perfectly incorporated as a key part of her identity, similar to wonderful main characters in Gideon the Ninth or The Night of Baba Yaga
-I'd never believe that this was a first book in a series, not because of missing information, but because of the author's security the story. The 'underground magical xyz' is present, but isn't presented in the way every other series featuring an 'underground magical xyz' seems to present their xyz. For example, my impression of the typical structure is 1) main character is introduced; 2) magical xyz is hinted at; 3) shennanigans; 4) magical xyz is fully introduced with some indulgent world-building paragraphs; 5) character is inducted into magical xyz. Mist in the Willows gives a most intriguing peek into the magical xyz that has left me wanting more in a manner that is much more satisfying (for me) than if the magical xyz was introduced in the book as is typical in the genre.

I do want to emphasize, especially for those here for the 'cozy horror' tag applied to the book, that the story is very much about PTSD and trauma. There's some ghostly gore, a really disturbing flashback, intense portrayals of supernaturally-spiked psychosis and schitzophrenic episodes, and even an event that to me definitely had the flavor of an on-page suicide attempt. Pointing these out isn't a critique from me; I thought that these were all necessary and well-done elements that served the story in delivering a bigger impact and more intense experience. I hope that those seeking "Over the Garden Wall" don't find themselves in an uncomfortable spot!

I'm concerned about how the art of the book might hinder its chances at wider readership. It almost turned me off (I'm so glad it didn't!), and while it didn't entirely detract from my experience, the quality of the whimsical, simple, slightly MS paint-y illustrations did not match, in my opinion, the caliber, quality, or serious subject matter of the book. I get the impression that these are a reflection of the author's love of the story and the creative process, which I appreciate in spirit. In practice, I think this book deserves a more 'professional' makeover. At least a cover that looks less... sorry, I'm going to say MS paint-y again.

In closing, I really hope this book finds more readers, especially fellow queer women.