Three things I like especially about this book:
1) It provided a lot of historical/cultural background, which is especially helpful to people of a non-Muslim background, who otherwise tend to feel left in the dark about a lot of aspects of this time and place in history.
2) If the historical record is silent on some aspects of Tahirih's life, the author was not afraid to state what would be the probable situation, based on what was typical in that time and place
3) It is not a strict narrative of Tahirih's life, but provides a wider history of the events that she was directly or indirectly associated with. For example, there is a whole chapter on events in the West in 1844 and 1848. This provides a depth of understanding that one would not normally obtain from a strict narrative.
It provides insights and deeper perspectives that not always found in books that are focused strictly on history.