I also started headlong into Only You Can Save Mankind. Terry Pratchett (PTerry) is currently the only writer I get emails when his American publisher (HarperCollins) releases a book, and I will be there within a couple of days to pick up my copy. This book marks the first HarperCollins release of Pratchett's decade+ year old Johnny Maxwell trilogy, the first kid's books he wrote. (His later kid's books include The Bromeliad Trilogy and his Discworld kid's books including Carnegie-winner The Amazing Maurice.) His kid's books are always just as enjoyable as his adult works, and usually only differ in length, protagonist age, and humor cleanliness. Only You... seems to prefigure some of his later humor (he loves the pronounce punctuation gag), but also provides an interesting contrast in that it has more of a "science fiction" feel than his later works... you could see this one actually happening in our world.
My biggest gripe is that it is simply too short and there will be a waiting period until the next rerelease. (It was cool to have The Bromeliad Trilogy released in a single volume.)