The title says it all -- this book's a short summary of the history behind the UN, the structure of the UN, the challenges facing it, and the avenues for future reform. It's a reasonably short read, although not quite one to breeze through, acronym-laden as it is (pretty much by necessity, given how the UN works).
As the author acknowledges, he's writing the book as a UN apologist. Yet at the same time, he doesn't hesitate to discuss the many times and ways the UN's fallen short of its original lofty goals. I tend to think it won't change many people's minds about the value or futility of the UN (it doesn't seem to have been written with an explicit goal of that sort), but at least it'll leave people better informed as to what the UN does and doesn't do, effectively and incompetently.