Less than 50 Tibetan thankas of the 11th to 14th centuries were studied till 1984. Since then, 300 new examples have emerged, which are the subject of this brilliantly illustrated book.
This is the most comprehensive book dedicated to addressing the evolution of thangkas from their earliest stages in Tibet until the 15th century, with tons of research by the author analyzing patterns in development and possible origins of styles (Indian and Nepali, for the post part, with eventual Chinese influences). The paintings chosen are all analyzed in great detail and compared to support the author's theories.
That said, this is not a good overall introduction to thangkas if that's what you're looking for. Most thangkas encountered today have been through an addition 500 years of mostly Chinese influence, so the iconography will be unfamiliar to casual viewers. Some of the explanations are technical and not for beginners to Tibetan lineages and cosmology.