Leon Uris's Trinity is generally considered to be one of the author's best works. For thematic reasons, it was one of my vacation readings.
Trinity is 900 pages long. The first 600 pages are superb, and just flew by. This is the portion of the book which covers the period, roughly, from the Great Famine through the Industrial Revolution, say, late 1830's through late 1880's. The characters were compelling, the storytelling was both exciting and colorful, and the book managed to be somehow intimate and sweepingly epic at once.
The genius of this part of this book is the way that Uris helps you understand why people behave the way they do, not by explaining it to you, but by showing you how it happens. People don't just wake up one day and do something dreadful to each other, it happens over a period of time, through untold zillions of tiny step by step actions and individual decisions, each of which seems simple and obvious and inevitable but they all add up.
The last 300 pages, covering roughly the first quarter of the 20th century, just didn't work for me.