I'm going to be honest. I LOVED Sigil-Beasts. Within the first two short paragraphs, Karalynn has already painted a picture of a vivid and interesting world. I'm imagining huge, rune-covered monsters, clashing in battles that resolve the fate of nations. This is a great premise because, not only does it have raw coolness factor, it puts a lot of emphasis on the individuals who are in charge of these beasts, which is great for story purposes.
The only place I think this story falls flat is in the later chapters. I like to try to craft each possible path of a gamebook to read like a story in it's own right--just as satisfying as if it were the only narrative path the author wrote. This is a steep challenge, and I'm not sure it's actually possible, but Sigil-Beasts falls pretty far short of that (admittedly high) mark. There were several story paths that I found very unsatisfying, with strange or unsettling endings that did not leave me feeling satisfied, like the story wasn't complete.
Just a little bit more on the system side: I felt like the rule-set was just the right length, neither too heavy nor too light, with each stat serving a valid purpose. And it was surprisingly clear and concise in it's presentation, especially for someone who (I assume?) is a first timer at this. (Is she a first timer? Has she done anything else I didn't know about?) That said, the balance later in the game got very screwy. I had trouble finding any path in which my dragon could win at the end, which was very frustrating. But I didn't explore exhaustively; I may just have made the wrong choices.
Another commenter suggested that Sigil-Beasts would have been better served as a straight narrative, either a short story or a novel. I don't want to scare the author away from writing gamebooks, because I think as a whole Sigil-Beasts was utterly fantastic, but I just want to throw out there, I would LOVE to see the novel that this story could become. Everything about the story, from the premise of the world to the cool house the main character lives in--built in the bones of his old dragon--to the moral ambiguities of the whole practice of raising and fighting the Sigil Beasts... there's so much richness and depth here.
I feel like the gamebook art could be honed a bit, as far as crafting the narrative paths and making each one satisfying, but the incredibly vivid and evocative setting and world (not to mention the clear, concise, and well-written ruleset) earned my respect. This one caught my eye from the beginning and remained one of my favorites throughout. It was a hard choice, but this came out as one of my top choices: Sigil-Beasts received one of my two votes.