There have been several early access books where the author needed to take a (well deserved) break, or be slow at delivering new chapters because of other revisionary work. This has sometimes meant a few months without new content, which I was OK with in that model.

In this model I could easily end up spending way more than than that, and every month I have to make a decision if it's worth cancelling.

Not only that you have another book on there that's "early access too" so you are juggling two books (and books are not trivial things to write and get finished/polished). So how am I to expect you'll get either done in a reasonable time.

So sorry, I'd be happy to give you a flat $30 for early access to the book and full access to the ebook when it's finished, but I'm not going to subscribe and hope it's in a good state prior to reaching that point in monthly fee.

This is despite I think this book is very much what I'm looking for as I'm wanting to get into 3d graphics, but I'll pass.

I also was confused by the model. The model you try to use is obvious for you but in order for people to understand you need to make an effort in communication, because most people are not used to it.

You have to be extra careful because people have fear of the unknown, so you need to also get rid of people's fears, you do it here("if you don't like it,just don't continue the subscription"), but you should do where people take the decision.

I think the model makes sense but you need to explain it in the buying page as a benefit they we will be getting. For example:

With this subscription you will be supporting the development of free open source libraries that will become yours forever.

Now I see that you display this information but I did not see it while being interested on the book, so it is not visible enough.

If I were you I would not try to train my customers, it is too much work, I would offer the book plus code plus 8 months of subscription for just $119!!

Then in the subscription I would give my customers something they can not get easily out of it, like the network connections and support with people interested in this area all around the world(the people that buy your book), so once they are subscribed, and they are used to it, they have to remain subscribed.

The value for me of those open source libraries is zero because I don't use them. Only if people use them, they can value it. You need to get people in the loop first.