The title of the book implies that it is a supplement by the use of the word "Hacks" and should be rated as such. Perhaps the author could have given this indication more, however, it is clear it is not an exhausting introduction to Ajax. For those of us who have been around in this industry long enough and understand the underlying technologies this is a good book as an extra with many tips for the web developer. Of course most of this can be obtained off the web and through forums, but it is always handy to have a reference like this on the shelf. I weighed this up with Ajax for Dummies and knew I would get more out of this book. Because the basics of Ajax is so simple to learn (suggest looking at the tutorials on the web) this book has more value. I have given it a 5 because I feel that overall it is worth more than just 4. However, note that it was published in 2006 and there are many more books coming out on this subject.