There is a veritable goldmine out there of books relating to the First World War; it depends on what you're looking for. Just from the top of my head:

For the outbreak of war, some new works include Christopher Clark's The Sleepwalkers: How Europe Went to War in 1914 (London: Allen Lane, 2012), Margaret Macmillan's The War that Ended Peace (London: Profile, 2013), Sean McMeekin's July 1914: Countdown to War (New York: Basic Books, 2013), and Max Hastings' Catastrophe: Europe Goes to War in 1914 (London: Collins, 2013). As with all history books, all of these have their strengths and weaknesses. For instance, I feel like Macmillan gives far too much credence to some events and circumstances (the Anglo-German Naval Arms Race, for instance, was over by 1912, and while it may have added to tensions in the prewar years it cannot be seen as a causative factor.) On the other hand, Max Hastings presents a rather orthodox, almost caricatured version of the Germans as Pickelhaube-wearing, goose-stepping warmongers; his is Fritz Fischer redux, without much of the nuance that made Fischer, if not correct, then at least worthy of consideration. Clark's book has rightly been praised for returning much of our attention to the Balkans (which is, after all, the immediate flashpoint of the war), but he tends to paint the Germans as passive non-actors who get caught up in affairs not of their making; in effect, he's an anti-Hastings. McMeekin's penchant for amusing bon-mots and his leaning on Russia is irritating. Of course, you should read these works and come to your own conclusions. Arguably, Clark and Macmillan are the most accessible (and Macmillan's Paris 1919 is a must-read about the postwar settlements in the Paris Peace Conference.) A slightly older book, Trevor Wilson's The Myriad Faces of War (London: Polity, 1986) is perhaps the bible of studies into the British political and military circumstances leading to and continuing throughout the war, but it's a monster length, and very sophisticated in its argument. Wilson, incidentally, is one of the greatest military historians of our time; with Robin Prior, his study of British command on the Western Front (unsurprisingly called Command on the Western Front) is unmatched, while their The Somme revolutionised First World War historiography, as it introduced the concept of the supremacy of artillery on the battlefield.

There are myriad books coming out (nearly as we speak!) about the war; Adam Tooze's new The Deluge promises to be good, and Lawrence Sondhaus' new study of the Great War at sea looks like it might be worthwhile. David Stevenson's 1914-1918 (which I think came out in the early 2000s) is a fantastic (if broad) overview of the whole war, and is worth a read. And here I'm really only scratching the surface. So there is far more to read than just Barbara Tuchman, and The Guns of August, while a defining work in its own right, has since been overtaken by some quite significant works. Happy reading!

I found 'Forgotten Voices of the Somme' (2009) to be a great book. It's excerpts from interviews with veterans of the battle, although they're overwhelmingly British plus a few German medical staff. Especially harrowing was one man's account of their first flamethrower attack, where 20 of his friends were burned to death.