It's quite hard to find books on World War I in Germany, at least compared to book on World War II. WWII completely overshadows it and often authors treat WWI just as a sort of prequel for WWII. I'm trying to remember if I learned anything about WWI in school...it cannot have been much. But WWII/Nazi Germany - huge topic. At least I was taught in depth about the Weimarer Republik, another chapter of German history that unfortunately gets the cold shoulder when it comes to books.
In The Pity of War, Niall Ferguson challenges popular theories about WWI, like the enthusiasm that is supposed to have greeted the outbreak of war or the allegedly superior armies of the British and French. He makes a good case against those theories and provides a lot of sources and in-depth research for his take on things. It was interesting to read such a different view of this war, a view that differs greatly from what is usually written about it.
I had to give myself a crash course in finances to better understand wide parts of the book. Economy plays a huge part in Ferguson's analysis. The most interesting chapters, for me at least, were those that investigated why soldiers fought in the war and why they continued to fight even in the horrible conditions of the trenches, why events like the Christmas truce didn't last.
So if you are looking for a book to learn about the Great War, this is a good place to start. You won't get the dry facts of what happened when and where, but you will get a good idea of why.
And since I'm on the subject, here are a couple of online sources I found very helpful when doing research on WWI:
First World War - multimedia history
WWI Document Archive - anything from diaries to photos. Including the Nicky-Willy telegrams
The Great War a collection of photos, postcards and other document...very graphic at times
a massive list of WWI links - I've spent hours exploring all those sites linked here