The beginning of a trilogy, so be forewarned. It sweeps off into a tale which, by the end of it, justifies the scope.
It opens with a formative incident from the hero Menelaus Montrose's childhood: his harsh mother forces him to delete anything she deems non-educational from his library --a thing of fabric -- including things he had worked quite hard to pay for.
The novel itself begins with his trip into space, where Menelaus is part of an interstellar expedition to an anti-matter star with a non-human artifact, the Monument. (Slower than light; this is an Einsteinian universe.) Between their departure from Earth and their actual arrival at the ship, where he slips into a legal loophole and performs an experiment on himself, to augment his intelligence. What else? The difficulties in deciphering the Monument are clear; even their carefully gathered collection of mathematical geniuses might not manage it.
It works, unfortunately, leading to only startling glimpses of the journey. Flashbacks to his past reveal the post-apocalyptic world in which he grew up and how he came to be on the crew -- he was an anomoly there, in some respects -- and his history as a lawyer, which is to say duelist.
He wakes to discover he is in the future, and had been taken to the star and back, and now the rest of the crew are effectively masters of the earth with the anti-matter they brought back. Such of them as did return. They also brought back a princess, the daughter of one of the dead men -- a fact that Menelaus derides as impossible before he starts to work out what happened. Flashes of memory help him start to piece together the jist of what happened, and what they have learned, and he starts to realize the Monument was more dangerous than they thought.
His old friend Blackie, leader of the survivors, has a plan to deal with that, and Menelaus does not approve. And the story builds from there to higher drama. And more drama clearly lies ahead.