Galileo's Dream is a pretty odd duck. Other than the recommendation from Ajax, I didn't know a damn thing about it before I cracked it open. I'd say more than half of the book is essentially a well-crafted novelization of the life of Galileo, focusing primarily on the composition of his major written works, and his interactions with the Church and the Inquisition attendant upon their publication. I know a bit about the relevant history, and for the most part Robinson hews pretty close, hitting the established marks, and quoting many of the documents that survive, from the letters written by Galileo's daughter [Robinson acknowledges Sobel in his afterword] to the flirtatious letters the aging, blind Galileo wrote to Alessandra Bocchineri Buonamici, his daughter-in-law's sister ... a married woman 30 years younger than him, to the letter from Cardinal Bellarmine setting down for Galileo's records the result of his first run-in with the Inquisition.
Anyway, though the book takes care to be historically accurate, it's not a history, and Robinson creates a mostly believable Galileo as a character. Where there is no history to guide, Robinson either follows Thucydides' lead in making "the speakers say what was in my opinion demanded of them by the various occasions," or invents entirely novel speeches and situations for sheer entertainment.
Of course, it's not just a historical fiction. At least I think we have to hope that it is not an authentic history. During the course of the novel Galileo suffers a number of syncopes, in which his consciousness is sent far forward in time when different factions of humanity are living on the moons of Jupiter. These factions have been meddling in time, trying to navigate a course between science and religion. Galileo is hopped up on drugs and taught about the science and history that has followed from his time, including his own life, which (in this little eddy of the timestream) ends with Galileo being burnt at the stake in the Campo dei Fiori, just as Bruno had. Although occasionally fed amnestic drugs, some of this knowledge remains accessible to him upon his return to his own time.
Given this change in the timeline, I find the least believable part of the book to be the idea that, having seen his death at the stake, Galileo goes on to write the Dialogue just as it was written in the real world. Maybe it's just me, but if the Pope personally acquainted me with his own argument about the relationship between God and the workings of the Universe, I would not cram that argument into the mouth of a character named Simplicio. Especially if I had foreknowledge of being burnt at the stake.
Anyway, in the neo-Classical future of the Galilean Moons, Galileo learns a lot of twaddle of future science. Manifolds and time dimensions and blah-bla-blah. Fortunately, most of that garbage is not important. Because these things in the future are Galileo's Dream.
And here I must briefly discourse again, because it must be the case that Galileo's Dream is inspired in part by Kepler's Dream, in which Kepler tells the story of space-travel to the Moon, giving him the opportunity to explore some very real scientific knowledge in a literary fashion, but also to invent lunar inhabitants and tell a story. But the biology of the creatures of Privolva and Subvolva are not really the point. The science is the point. I think.
Anyway, the pseudoscientific twaddle of the humans of Ganymede and Europa is not the point. The point of Galileo's dreams is the discourse about the relationship of gods, man, religion, knowledge, and science, and these points inform Galileo's actions back in the real world. Unfortunately, I think this discourse about these relationships is rather confused, and it's hard to see what effect they have on Galileo's life, and why it would have those effects.
But Galileo, who did NOT get burned at the stake, gets bumped onto the timeline we all know and love.
A for historical fiction (especially if you aren't familiar with the story)
A for historical accuracy (apart from the stuff that isn't)
A- for a Galileo character
B+ for psychoanalysis of Galileo
B for twaddle
C- for a meaningful resolution to whatever philosophical question is being explored.
Special note for time travellers... Set your dial to 30 July 1947 and enjoy the English premiere of Berthold Brecht's Life of Galileo at the Coronet Theatre in Los Angeles, with Charles Laughton (who helped with the translation) as Galileo.