First, the sixth edition is the first I've read, so I can't speak to any changes. Instead, my review is focused on the book as a first-timer reader to the 'series.'
Historical information is also included, which I found to be very interesting when it came up, as well as implementation-specific functionality, that has limited use at this time (and as such, I personally found it distracting, and began skimming over later instances, but it's still nice that it's provided).
The jQuery information is around 60 pages of content, covers version 1.4, and also includes a bit about jQueryUI (a very little bit). It's quite refreshing to see jQuery included in the book, but as noted initially, if you're looking at focusing just on using a library, it may be better to get a resource focused on just that.
The second part is approximately 40% of the book.
The third and fourth parts are similar to O'Reilly's other reference books, and are therefore fairly detailed, with examples included. Depending upon your preference, you may find the reference valuable, or prefer searching online. The examples included give the book a slight advantage over the average Web site. Honestly, I generally prefer using online resources, so I don't see myself consulting these later parts very often, if at all.
Finally we come to the actual book itself. I received an electronic copy of the book, through the O'Reilly Blogger Review Program, so I can't speak to the quality of a physical copy. However, in the past I have generally found O'Reilly books to be well made, with bindings that last.
And now comes the rating.