I've read most, or perhaps all, of the other Gordianus the Finder books, though it's been a while. I enjoyed them all very much.
This one is a prequel, set in 92 BC when Gordianus is a young man. I think it starts off rather slowly; I was thinking I might not give it such a high rating.
It seemed like a collection of short stories rather than a novel (in fact, several of the chapters were first published as short stories), it didn't seem very coherent, and the individual mysteries seemed to be over before they started. It didn't seem plausible to me that Gordianus, as this young sprout, unknown to anyone in these exotic scenes, would be allowed to exam the evidence in all these mysteries, or listened to. However, in the second half the book picked up steam and came together; there is a theme to the entire book after all, and it the end is feels complete and satisyfing-- and links to the rest of the series.
One of the mysteries Gordianus solves is "the second riddle of the Sphinx", which, however, is quite different from the second riddle of the sphinx that can be found in some versions of the myth. That particular chapter seemed to be one of the less plausible ones, but one of the more interesting ones at the same time.
I think if you like the other Gordianus stories you'll like this one; if you're really a mystery buff and like to solve the mysteries, you might not like it quite as much. It's well written, flows along very nicely, and mostly it gives you insights into the ancient world that would be hard to come by any other way. Some of it is speculative, of course, but it's not Dan Brown, OK? It's got to be at least 60%, 79=0% true, easy, and there are some suggested readings in the back.
If you find yourself skeptical about any of the historical facts, you can Google them; in fact, that's one of the great things about this series. Like, OK, you've heard of the Hanging Gardens or the Great Pyramid, maybe you know 25 words' worth about it; the great thing is not that you learn another 50 words, or 250 words, about that thing, but that you get much more of a sense of it, and interest in it, and can go learn more if you want to.
I found this book, like the rest of the series, interesting, entertaining, and educational.