It is a long time since I have read that book, not since I wrote the webpage, so I can't comment on its quality or usefulness with certainty.
However, for me, I remember the book lacking the appearance of stature, so it was hard to accept his opinions as being original, or if original, sound. The author in the intro writes he had made just two visits to Aghtamar, his first being in 1987 which triggered his interest in Armenian architecture. The second was 1988, and it resulted in this book. Such a short time period (and with a very a limited range of monuments visited: nothing in Armenia, and not even the architecturally similar Soradir church in Turkey) does not seem to me enough time for anyone to gain an understanding of the subject deep enough to write a useful book.
The photographs in the book are very poor in quality. Inexplicably, they were not taken by the author but by a photographer who accompanied the author to the island. That sort of thing might be expected up to the 1950s and 1960s, when some academics still considered photography to be beneath them and to be manual work. The only good reason to see this practice in the late 1980s would be if specialist photography was involved - but there is none of that in this book. There are not even any decent close-ups of the reliefs, or wide angle shots of the interior, that might justify having someone who has specialist lenses.
Newer books include one by A.E. Redgate which I have not read so can't comment on.
There is a Turkish photobook of good-quality photographs that reuses parts of Mnatsakanian's text, and another Turkish book is a small paperback volume by Sinan Kilic, propagandistic in tone and content, but which mentions the site's post-"restoration" condition, including the buildings recently excavated and smaller finds like Urartian inscribed stones.