It should be highlighted that there are two different aspects of thangka painting - iconography and methods. The book mentioned in the question describes the methods only and such knowledge should not be considered as a part of some Buddhist teachings. Iconography, however, is an important aspect of Tibetan Buddhism used in thankgas, statues and visualisation practices.
It is not straightforward to get access to texts describing the details of a given Buddha form. Tibetan Buddhism is based on a direct (usually oral) transmission and some texts are allowed to be used only after the authorization from the teacher. For example, I did receive the authorization to practise the meditation on Vajrasattva and I totally agree that the Dorje should be in the right hand of the deity. (Admittedly, this very detail can easily be checked on Google images by typing in 'Vajrasattva' or 'Dorje Sempa'.)
Iconography of Yidams (Buddha forms) is very strict and systematic. Every detail of a form has a meaning and thus if one wants to use the thankga (or a statue) for practise purposes, it has to be painted correctly. In practice it means that the artist should have a proper training - possibly a direct transmission from another artist and/or meditation master. I've also heard that the best if the artist is meditating on the Buddha form while he is painting it.
I know some people who are buying thangkas and statues in Nepal to be later distributed in European Buddhist centres. They said that they never buy ready-made thangkas as most probably they are made for the tourist market that is for the customers that use them as a decoration rather than for a proper Yidam practice. The most reliable way of obtaining a genuine thangka is to find a renowned artist and commission a specific thangka from him. Then one can be sure that it will be done with all the due care as the artist knows that he can spend days or weeks on one painting because he will be paid for his hard work immediately. Afterwards the thankga should be checked by some master who has a transmission on that Buddha form to confirm whether it indeed has been done correctly. One may wonder why it is such a hassle. Well, either one wants to have a nice decoration on the wall or one wants to practise on Yidams. There is nothing wrong with having some Buddha picture on the wall if one finds it inspiring and calm-giving. If the thangka lacks some details, no harm is actually done. If one wants to practise on Yidam, however, the Yidam itself has to be painted with all the fine details. But then it presumably means that one obtained some authorization and explanations from the teacher who is also able to check the quality of the thangka.