This is an amazing book to read. I read it while making the transition from extract to all-grain brewing. Actually that's not quite true, I bought this book and it inspired me to switch to all-grain brewing, which I have definitely not regretted.
What this book has is lots of all-grain recipes, which you can brew as they stand, or use as starting points for your own ideas. Mosher gives lots of hints about where you could take different recipes, and the book is packed with tips about how to combine ingredients to make great tasting beer. It's not exactly a book about how to design your own recipes, but it will get you thinking in that direction, and if designing your own recipes from scratch is your aim, this book is be a great place to start.
1. Keep in mind that the gallons means US gallons.
2. It seems like somewhere along the way some of the numbers in the book got mixed up. Before you brew a recipe I recommend checking that the numbers all add up. I.e. that the amount of grain used should give the OG the recipe says, and if not check the errata on Mosher's website. This might seem like a big defect in a book of beer recipes, but it really isn't. The value of this book is the wisdom it imparts, and working out the correct numbers is not to difficult.