It is a book that you should buy, not to download. Of course it is possible to download it somewhere, but it is based on wx2.6.x
raph has already commented it, but i would like to add that this book is edited as part of the Prentice Hall Professional Technical Reference (phptr.com, just the site from where the download is available), inside the Bruce Perens' Open Source Series. This, and all the other titles included there (most of them also available for downloading), are under the open source license, so they are legal to copy, modify, and redistribute. The electronic versions of the books are placed on the web a few months after the print versions hit stores, and there is no legal issue with downloading these e-books.
Regards.