IMHO the existence of a book which provides detailed guidance and instruction on the use of a tool, but which costs money, does not negate the FLOSS nature of the product itself.
Though admittedly a weak analogy,
gcc meets all the criteria for Free Software. But to really master C++ or C you will inevitably need to read some books on the subject, and many of those are not available for free.
I can see nothing, other than economics and the lack of comparable knowledge, to prevent someone else writing an excellent book on ANTLR which both competes with the Definitive Reference and is made available for free. This would not make the ANTLR software any more 'free'.
This approach, of providing the software for free and selling a definitive user guide, is just another approach to monetizing Open Source development. You are not forced to purchase the book in order to use the software.
You could invest $27 of your time in studying the subject matter and ANTLR source code to build the same level of expertise you can gain by purchasing the book ... but I doubt you'd get very far in a cost/benefits sense.
That said, the author does not appear to have provided any substantive FREE documentation beyond links to the book. However the e-book is non-DRM and simply googling the book's title and 'PDF' rapidly finds many places where it is available to download. I don't know the legality of those sites, and I suspect none are authorized.