An Instance of the Fingerpost by Iain Pears, 1998.

I was very pleased to complete this book which, at just under 700 pages, is the longest that I have undertaken recently. The novel is set in 1660s Oxford and concerns itself with religion, politics (Cromwell and Charles II), medicine and science. The book consists of four parts, each of which is told from a different character's perspective: Marco da Cola, a Catholic traveller from Venice; John Prescott, son of a supposed traitor to the Royalist cause; John Wallis, a code breaker who worked for both Cromwell and Charles II, and lastly, Anthony Wood, an Oxford historian and man of learning. Most of the characters in the book were real people but the exceptions, James Prescott and Sarah Blundy, were based on actual figures of the day. There is a list of the individuals at the end of the book with a potted biography of each.
The main event of the book is the death of Dr Grove, a fellow of New College, Oxford, who is discovered to have been murdered. Linking all the stories and the murder is Sarah Blundy who, with her mother and deceased father, Anne and Ned Blundy, was a Leveller during Cromwell's time. Sarah and her mother are now down on their luck and living in poverty. Sarah is a servant for several characters in the book while her mother, near to death, is treated and visited by other characters. An amusing description is made by Marco da Cola of a play he goes to see which the reader deduces is Shakespeare's King Lear.
The revelations and explanations are made in the last section of the book by honest historian, Anthony Wood, and may impell the reader to revisit the first three sections of the book if not put off by the length of the whole. A prospective reader would probably need some interest in the complexities of the 17th century, but there is a mystery at the heart of the book which does achieve resolution by the finish.