The Game Inventor's Guidebook is a decent and breezy--if outdated--guide to how to go from a games hobbyist to a games inventor. Tinsman has the games business chops, as the acquisitions guy for Wizards of the Coast, he worked on Magic: the Gathering and Curses and played about 150 new games a year. This book is his attempt to look inside the business of games, and help people break in. As with most business how-tos, it's only a fraction of what the author knows, but's it an important fraction. This book provides a basic overview of the major players and types of companies in the business: I enjoyed the interview with Rob Knizia, and how hopeless publish an adult casual game with a major company seems. Just knowing how to think about agents and publishers is useful, even if the specific lists are a decade old.

What's most useful is the knowledge that 90% of games are thrown away unread. Many amateurs tend to design heartbreakers ("It's just like Monopoly, but...") or over-complicated monsters that are too expensive to produce and impossible to learn. Creativity and elegance are things that any aspiring game designer should strive for, along with an awareness that this is not career for people in it solely for the money.

Ultimately, the problem is that this is a perishable book. Tinsman isn't quite insightful enough to provide eternal wisdom for a changing industry.
The two biggest changes that an updated version would have to take into account are the rise of euro-style games in the US (They're mentioned, along with Spiel de Jahres, but they've exploded since then), and Kickstarter/crowd-funding as a way to publish indie games.