Sugar Dark. Neither very sweet. Neither very dark. If you don't consider the topic about death dark.
Seeing as how this LN doesn't have a review, and I prefer to write reviews for stuff that have few/no reviews, here it is.
Do note it contains my raw, biased opinion, but no spoilers because there isn't much to spoil. Having read the manga and this, I will compare them but after the review of the novel itself.
Sugar Dark is an interesting story to read. It may say 3 chapters, but each chapter is divided into sub-chapters and all in all will probably take about 4 hours to finish.
That said, the story itself is pretty interesting. It isn't light hearted but nor is it dead serious; just that the MC doesn't have flowers inside his head. If you're looking for something deep in the abyss, this isn't the story for you. But if you're looking for a change of pace from the typical shounen story, this may just suit you.
The characters aren't a lot, which makes remembering them easy, and they do play their designated roles perfectly. Character development was there for the characters that mattered and that was pleasing. Now onto the plot. The story isn't particularly long (its only 1 volume) but it progresses nicely. There're isn't any particular repetitive things that make it dragged out. The way the translator(s) did the structuring of the sentences reflected a more mature tone, which was befitting of the story.
And finally the ending. It was bittersweet. It felt satisfying to read. I'd rate it higher if the canon continued, but that said, I'd still recommend this story if you're looking for a seinen that isn't so serious.
On to the comparison between the manga and the novel. If you prefer pictures, you'll like the manga. If you prefer text, you'll prefer the novel. It sounds obvious but I'll explain. The manga doesn't show his thought process in detail, the novel does. But the manga is easier to follow and visualise because pictures.
Now which do I prefer? The novel, but only because I'm a novel person. When I read the manga I was still pretty fresh so the mature part hit me because its pretty graphic.
I guess that's about it. Hope it helped someone decide whether or not to read it.