The book is not fun sledding. He has plenty of asides complaining about how Catholic or Protestant scholarship has tended to read their premises into the past (no doubt true enough), and he seems compelled to pay homage to current pieties about searching for non-patriarchal Christianity and the value of Gnostic writings. And the attitude that every religious document is equally true/false depending on the viewpoint of the devotee gets pretty wearisome after a while. On the other hand, it is fun to watch him dynamite the fashionable claims that the heroine is always Mary Magdalene.
But he succeeds in dredging up as much as he can describing Marian interest and devotion from an era with little documentation about her: the dark years when persecution meant little was circulated, and the subsequent century when the church fathers mostly wrote about doctrinal issues.
From about 145 AD we have Protevangelium of James, which is an extremely influential pious fiction about the life of Mary. The names of Mary's parents come from this. It mentions a rock that Mary rested on on her journey, over which a large church was later built, and claims that Jesus was born in a cave in between Bethlehem and Jerusalem (contra Luke)--a cave still celebrated in art. There's also an interesting scene in which Joseph, looking for a midwife, notices that time has stood still.
There's nothing about Mary being an intercessor here, but it is very extensive and Mary-focused--and fairly early. Well after the last of the New Testament, but still early.
There are some Gnostic (he prefers "heterodox") documents as well, in which typically Mary is a teacher of secret wisdom. I'd bet Mary's secret name was Sophia.
There's Six Books Dormition Apocryphon in which Mary is teacher, intercessor, healer, and which specifies some liturgical rites for her festivals.
And there are shrines (not always easy to date) and some homilies and the Pulcheria vs Nestorius conflict. All these point to popular pre-existing devotion to Mary as intercessor, as well as exemplar.
Why, then, do the church fathers not mention any of this? Two obvious possibilities: They didn't generally write about liturgy and devotional practice (some exceptions), and some of the devotees were decidedly fringe.
A big problem with all such research is the scarcity of documents. You may have to reconstruct what A said from what B said that C said about A, and maybe correlate that with what D said about what E claimed about A. And documents that do exist are often fragmentary, copies don't entirely match, and so on.
Plus, if you are trying to figure out what the orthodox were doing on the basis of Gnostic writings, I'd think the best you generally get is "well, this was in the air somehow."
BTW, if a document addresses aeons and levels of creation and secret passwords for getting past spiritual levels, I classify it as Gnostic. From philosophical and literary points of view these are entirely different things from the orthodox canon, setting aside claims about infallibility.