I'll preface this by saying that I don't want to be mean and just give you honest feedback. I haven't read either of your books and just looked at the amazon page you linked, look at the cover and read the blurb for The Golden Seed, and looked at https://gabrielgambetta.com/computer-graphics-from-scratch/, looked at the cover, skimmed the table of contents and read the paragraph under the title. This is of course entirely personal feedback based on how I feel about both of your books.

For "The Golden Seed", both the cover and the blurb seem "too much", like the kind of book you encounter by the dozen in a bookstore. It's also a fiction book, which appeals less to me because I can either ask people in my family for recommendations (there are a few big readers) and then have something to talk about, or read well-known books (for example, I've read The Three-Body Problem recently, after hearing about it 4 or 5 times here and really liked it).

On the other hand, Computer Graphics from Scratch attracts me more: its cover is in the same style as "Automate the Boring Stuff with Python", of which I've heard great things about; the "from scratch" appeals to me because I like the idea of building things from scratch; the table of contents has an entire part on raytracing, and the book seems oriented at beginners, which I like because I previously tried and failed to build a raytracer following the raytracer in one weekend project, and I think I could achieve it with this book. It's a technical book, which I'll value more because it feels like I'm building a skill. I also have a friend that made a few graphics experiments as a hobby and this could be a great conversation topic.

Again, this is very personal feedback but I hope it can help you understand why some people might buy one and not the other.