I have read Hart's Gallipoli, and while his style is not my favorite, I would like to know more about specific battles, such as Verdun and the Somme. I've read Keegan's one volume history, but I'm looking for things that go more in depth in regard to specific battles.
Paul Jankowski's Verdun and Alistair Horne's The Price of Glory are both excellent sources for Verdun; steer clear of John Mosier's book on Verdun.
William Philpot's Bloody Victory and Peter Hart's The Somme are both excellent accounts of the Somme.
John Terraine's Road To Passchendaele is a good explanation of the campaign and the rationale behind it. Nick Lloyd's Hundred Days and John Terraine's To Win A War are both excellent accounts of the Hundred Days Offensives.