It's for a good reason though: they've been adding even more content to the book than originally planned. From the original blog post:
Unfortunately we have had a delay in production due to the fact we couldn’t decide what to leave out and as a result we have had to add some bonus material to the book. This has pushed us past our deadline and we are now looking to have the book released in November
The delay is only a couple of months and should result in an even better finished result. If you haven't done so, you can still preorder the book directly from Cartoon Saloon.
That's not all, as according to Amazon UK, the home release of the film in the UK and Ireland now has a release date: November 2nd 2015. Currently the listing doesn't have any details of what the release will contain. However we'd hope for it to at least to match the existing US Region A Combo release, which contains some excellent features including an essential commentary from Tomm Moore. You can preorder your copy below.