I agree that the thematic arrangement is a great idea, I find the selections heavily edited. I would have liked to have seen less selections and more of the actually texts presented. Maybe they could have done a two volume set.
This very lengthy book arranges primary source readings by a vast number of mystics thematically. A very good study for someone who has been introduced to the mystics and is seeking to expand their knowledge. The thematic arrangement sets this book apart from other anthologies of Christian mysticism.