I highly recommend Jeff and Michael Shaara. Michael Shaara wrote The Killer Angels and excellent history of the battle of Gettysburg and his son, Jeff, wrote a great history of WW2. Technically the books are fiction but are very accurate and well written.
Similarly, I recommend the Flashman series by George MacDonald Fraser. These focus on Victorian Britain and, although they are fiction as well, are very historically accurate and a real pleasure to read.