For many years DDTM was the recommended book for an introduction to Neopaganism. At the time, it was the only one. It's still a recommended read but there are other titles now too.
There is a great deal of references to Wicca mainly because Wicca has had the most influence on modern pagan practices than any other religion. Even so, the book is even-handed in its overview of the Neopagan movement. Early editions may seem outdated as the movement has grown in the past few decades (it first came out in 1979). There have been recent editions but I don't know how accurate they are and they have other's input as Margot died several years ago.
But yes, she was a reputable source. She was a Trad Wiccan initiate and well-regarded in the Pagan community. Sidenote, she was also an NPR reporter.
As to other books that are often now recommended along with DDTM, some are The Path of Paganism by John Beckett, A Million And One Gods by Page DuBois, and A World Full of Gods: An Inquiry Into Polytheism by John Michael Greer.