Not diffucult to read, per se, given that Catton was a professional journalist, not a professional historian. And it is aimed at about a US 7th Grade reading level, so there should be no difficulty in that regard.
However, the book is not well understood or appreciated unless the reader already has a pretty solid grounding in Civil War history. The roles of the many generals and politicians, to say nothing of the many similar battles, are confusing unless you already know the essential background.
This book sat on the shelf of the family library unread by me for many years. It was not interesting or attractive to me until after I had more thoroughly educated myself on the war in the East.