I'll let my oft used History degree kick in for a moment here
The two books I'd go to for an overview of WW1 are David Fromkin's "Europe's Last Summer." It is a fantastic take on the intricacy of how Europe devolved into war, and very much more than the conventional explanation. For the millitary history, a great go to general volume is Keegan's "The First World War."
For World War 2, I tend to divide the overview books into Europe/The Pacific. But, if you want a really easily readable view of the whole war, I'd look at Stokesbury's "A Short History of World War 2." For Europe, I'd look hard at Hasting's "Overlord" and "Inferno" which cover the Western front from D-Day foreward. For the Pacific as a whole, I'd strongly suggest Toland's "The Rising Sun."
I have a huge history library, so I'd be happy to provide suggestions for anyone who wants one on a specific topic.