I liked the premise and writing of Gemina well enough that I should have given it a five-star rating. But I didn’t. The sequel to Illuminae is told in the same unconventional manner, but that isn’t the problem.
Book design is the reason I downgraded Gemina from five stars down to four. For starters, there is too much use of gray colored font against dark page background. I started out straining to read it, but a few hundred pages in I gave up trying and skipped past it when it occurred. If that was the only problem, the book would have still been a five star read for me. The bigger problem was toward the end of the book, when there was interaction between two alternate universes. For 50-60 pages, the story is presented in page pairs, with one page detailing the action in one universe and the other page telling it from the other. Each page pair had about 75-80% of the same material on each page. This section brought the story pace to a near halt and lessened my enjoyment of the book.
One other potential issue I saw was that the voices of the two main characters didn’t seem to be differentiated that much from the two main characters in Illuminae. It’s been several months since I read the first book though, so I could be wrong.
Despite the problems, I still gave Gemina four stars, and will definitely read the next installment in the series.