I am interested in start exploring Away 3D, and I saw the 2 new books that were published in the last couple of months. However, I am more interested in start exploring version 4.0.

So, how relevant are those books to version 4.0?

unfortunately in my experience at the moment the 3.6 books have quite a few demos that do not work in 4.0. From watching Fabrice and Richard and the other devs posts, a lot of the classes in 3.6 that are missing from in 4.0 are being ported over in the near future.
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I found that kind of frustrating at first but it forced me to “read the code” more and I feel I have a better grasp on how Away3D 4.0 works now. So, I recommend the 3.6 books as a primer, but don’t expect them to translate directly yet at all