I've found that the JavaScript Tools Guide CS6 PDF is not available for download from the Adobe website. It is only available as part of the ExtendScript Toolkit CS6 (ESTK 3.8).
ESTK 3.8 is not available for download from the Adobe website either, but is included in all Adobe CS6 products.
If you have an Adobe CS6 product installed on Windows, the JavaScript Tools Guide CS6 PDF can be found here: C:\Program Files (x86)\Adobe\Adobe Utilities - CS6\ExtendScript Toolkit CS6\SDK\JavaScript Tools Guide CS6.pdf