If this sentiment seems trite to you, you may find little of real substance here, though as a schoolteacher who often wishes he were rich and famous (I answered YES to almost every question on the first page), I certainly need vivid reminders such as this book that the rich and famous aren't significantly happier than the rest of us - or if they are, it may have very little to do with their wealth and fame. From that point of view, it's a soothing balm for the would-be materialist's aching soul. Success, money, fame, houses, yachts, soft-furnishings, shoes - none of these things will make you happy - they can't.
Having said that, James' editor should have sat down with him and forced him to re-write it. There are jarring inconsistencies of tone (James refers to himself as his readers' 'heroic mind tourist', and says 'Err, see what you mean mate' in an aside), inaccuracies of punctuation ('as my mother said shortly before she died when my wife was describing her birth plan' - how very unfortunate that she should have died at that moment!), and, as has already been noted, broad unsupported statements that support his arguments when their opposites could equally easily be posited.
I love the portrait of the deeply unhappy multi-millionaire contrasted with the taxi-driver in the first chapter, but it's just too easy. I bet there are loads of unhappy taxi-drivers, and there may even be one or two well-balanced, fulfilled billionaires too, mightn't there?