same here. Something I've been wanting to learn in a while being fascinated by computational linguistics.

only the REST api is open source.

I see the book as a way to sell their API for a service for which everything (algorithm and methods) are obfuscated (server side).

I expected much insights for helping me on a project, I had less informations and more noise than what I already knew.

What on earth do you mean?

The entirety of NLTK is FOSS originally produced by a university. Even if NLTK had some sort of hosted version (and I was unaware that if they do, as your post seems to claim), you could always go D/L the whole source (https://github.com/nltk/nltk ) and use it as you see fit (it's even Apache Licensed).