John Banville's, "Dr. Copernicus", was excellent in its entirety, and sections of the work were exceptional. "Kepler" which is a sequel in a Historical sense, may not match the former for its consistency of excellence, however it is still a very good novel, it takes the work of Copernicus another step, and is a piece of work that is 5 star material when compared to much of contemporary writing. The four star ranking is only relative to, "Dr. Copernicus".
The idea of whether these early stargazers believed their work documented truth or merely supported what they observed is taken a step further with Kepler and his work. When Kepler and his peers were working, mathematical proof was becoming the essence of what they would eventually publish. Work that appeared to explain what was seen was no longer enough, proving it to oneself and one's peers was the new test. One of the great enigmas that Kepler sought to solve was the orbit of Mercury. His findings were to change the Copernican view of the Universe, while Galileo was extending the very reach of it.
The science, and the math employed are raised a couple of steps from the previous novel, and are part of why I liked this work less. Understanding complex ideas should not be brought down to such simplistic levels so that no effort to understand is required, and whatever is learned is of little use as it relates to the true and complete idea. I always enjoy a writer that can explain complex theory in a manner that allows an inquisitive mind to be challenged, and the science enjoyed. In "Kepler", this did not happen the majority of the time. So the reader must just take on faith what is said, or study some pretty advanced geometry.
"Kepler" also does not have characters that gain your empathy, and at times you may struggle to find them interesting. Money, position, prestige, all conspire to intrude upon the greater goals, and I found them distracting. However the treatment of the Church and the Schism, and the effect on Kepler and his work, was very well done, interesting, and demonstrated the Church's constant interference with the scientists they were terrified of, for their work would undermine the Church's long taught fictions.
Mr. Banville is a writer of remarkable skill. I am reading the fourth of his books, and the quality of writing, his skill with a pen is never an issue. How he presents his story may or may not be enjoyable to a given reader, but all will appreciate the skill with which he writes.