Hmm, I am very conflicted here.
I deeply enjoy sooo many tweets and threads made by Jeannette Ng - especially, when it comes to world building, which brought me to following her on twitter in the first place. And also made me pick up a copy of her book, as soon as I found the time and money.
While her tweets convince me all the time, the book did not.
Let's start with what I truly liked about the book, as the parts that didn't feel so great are pretty spoiler heavy.

There are concepts in "under the pendulum sun", that touched me as they were so fantastic, seemed clever or colorful. I love the idea of the seawhale, that holds a sea and smaller fishes inside of it. Also the moon being a fish, that can be lured to change it's path - brilliant! I can see that in front of my eyes right away and really feel, that there is a world that differs from ours.
Also some parts of the story, around the middle, were nicely paced and got me to read longer than I should before going to sleep.

Unfortunately most of the book couldn't keep this speed up, maaany chapters feel like a drag. When I finished the book, I couldn't fend of the feeling, that I would have been way more enjoyable, being 200 pages long instead of 400.
I guess this feeling mostly stemmed from the really shallow charactes navigating at turtle speed, while they encountered "plot twists" that one could unfortunately see coming ages ago. Most of the "plot twists" were painfully obvious after reading the first chapters, to be honest. I was waiting for a proper twist, SOMETHING, anything to surprise me... but nope. I had some hopes for the last chapters, but there was nothing. Or even less, as one didn't even find out more about the allegedly "sooo different" fairy world.
As far as world-building goes, I was even more disappointed. It's one thing to build a complex world and another to just throw in some information here and there, start something and never finish it. I know, it's what a lot of big, successful writers do, but in my opinion it's disappointing and lazy.
Also, when there is a "new" world to be build and populated... how can you keep so many concepts that don't even make sense in our (pretty western) human world? Let's just take gender as an example. It really made my eye twitch, that everyone in fairyland is being gendered male or female, and not one of the persons and creatures questions that? Especially when everyone is already ridiculing the human missionary? What a I was really speechless when there were angles who were also gendered and didn't question it? Angles, the most non-gendered beings in the whole bible? What? This was really a bummer. One also didn't get any information about changelings, what they are, how they're different from humans. Nothing.
Well, one could say, that as fairies never give you real information, so did the author, but well no. For me as a reader this is just lazy and annoying.

When I am completely honest, I couldn't stand the premise of the book.

(this is a serious spoiler!)

When I read the first pages, I was already rolling my eyes by the relationship between brother and sister. Well, it's a take on Brüderchen und Schwesterchen, I guess, with a good portion of Kaori Yuki's incest fantasies... but it's still weird. Not, because I condemn incest per se. I'm far from it. But in this case, the incest thingy being kind of the main plot twist, while still being obvious from the first chapters on,... while both behaved just cringe-worthy and were characterized so shallow, that I couldn't see why on earth they would love each other. I felt like like reading some really Mary Sue-y OC fanfics, whenever both talked/thought about their love for each other.
Also what's with the "when there are two persons of opposite (binary) genders in one household, they have to fall in love with each other"-trope? In a feminist writer's story? Where did this come from. Just no.


One minor pet peeve I had with the book was, that at times it felt like a show-off. Excerpts from bilical and academic texts concerning the bible were sometimes necessary, as well as dated vocabulary, suddenly parts of German fairytales in German (no problem for me, but why? and not too nice for non-german-speakers)... but I often felt like the author just wants to show off how much she knows about the subject. As especially the sometimes quite long texts at the beginning of each and every darn chapter sometimes contained valuable information, but very often didn't and just slowed the reading down even more, I could really have done without them.

My expectations for world-building were really not met by this book, neither was I happy with the weird (imho not in a goth way) uber-straight love story. But there were attempts of potentially exiting story-telling and inventing of new concepts.
I hope that's just the way first novels (especially when as long as this book) are just not there yet, but that the next book will be a bit more convincing. I'll definitely keep my eyes open for it.