I've got the green book, "Modern Bridge Bidding for the 21st Century". It's a description of a complete off-the-shelf bidding system which incorporates 2/1 GF, i.e. there are also sections on 1NT openings, strong openings, overcalls etc. The system works fine most of the time and I have used it with some friends as a kind of baseline on which we can all agree. It does have a few holes, and with a regular partner you'd want to develop it a bit further, maybe taking some ideas from the purple book. It's poorly written. Hardy has an annoying habit of repeating himself, so you'll get an introductory paragraph which explains the system, then a longer paragraph which says exactly the same thing in more words. That's sometimes confusing, because you're expecting to be told something new. On the other hand, there are loads of examples and exercises which clarify things a great deal. I'd recommend the book to someone who's not really playing any consistent system already, and wants to be able to say "I play X". If you're already playing another system complete with bells and whistles and want to try 2/1, you might find this book frustrating to read.