The web page for the SPL has a link to an ePub copy. But ePub seems more of an export format, and isn't really editable outside of Linux-like tools. Is there a version of the book for Apple Pages or the like? I want to prototype an idea as a change to the documentation first.
I don't think that the 'source' of the book is Open.
You can unzip the ePub to get the HTML sources.