There are better books - Willis, Skehan, Ellis, Edwards and Willis. I read the original of this book, and the new updated version is definitely better. Nunan a lot of the time when he thinks he's talking about TBL is in fact really talking about PPP. I have read almost everything on TBLT and Nunan in my view is a very minor figure rather than one of the important researchers/writers in this area of methodology. I bet in the future his next article or book won't be on this subject. So based on all the above I give it 3 and say there are better books out there.