The book is as much about Margrethe Mather and her personal and artistic relationship to Weston as it is about Weston and his evolution as an artist. Warren presents as good a picture of the enigmatic Mather as possible given the available records, except that I think a more critical aesthetic analysis would have been helpful in sorting out how Mather and Weston influenced each other. Looking at their pictures side by side when both were working primarily in the gauzy pictorialist style, Mather seems to me as good if not better than Weston in many instances, particularly in regard to the portraits. Mather's portraits show a real connection to the personalities she photographed while Weston's from that period often seem like caricatures.
The works that Weston at the time portrayed as avant garde usually involved unconventional compositional balance in a manner which may have been more Mather's creation than his own. Ironically, he presented his work at that time as quintessentially photographic though it was obviously grounded in 19th Century painterly values. The cover photo on Warren's book looks back even further to the stylistic preference of the period for Japanese woodblock prints. The message those pictures by Weston mainly convey is one of conflicted intentions.
While there was some aesthetic parity evident between the two artists, Mather's work doesn't seem characterized by internal conflict. She clearly didn't share Weston's level of ambition, and it seems her identity was not wholly tied to her art as was the case with Weston. While still together with Weston she seemed content to remain in the soft-focus mode when many of her peers in the wider world were moving toward the Straight Photography model. A good example of this is the work entitled "Water Lily" from 1922 which is transparently derivative from Baron Adolf De Meyer's still life compositions from 1908. It seems odd that Warren chose to include the picture in her book without commenting on that connection and its significance. Later, Mather moved away from the soft-focus style. She did a compelling series of close-up patterns of shells, cigarettes and other man-made and natural objects. Her pictures of hands and feet are evocative of the work of Stieglitz and a flower shot suggests the influence of Imogen Cunningham.
Given the history Warren presents of Weston, Mather and their circle of friends it doesn't seem surprising that Weston made the leap he did into a new life and a new photographic style, except for the fact that he was 37 years old at the time. It does seem extraordinary that he was able to successfully recreate his career as an artist at that point, though not with the rapidity that he probably first imagined. He did some exceedingly nice work in Mexico, but achieved little real immediate financial or critical success. That would come much more gradually as he abandoned Modotti and the Mexican adventure and returned to California, ultimately hooking up with Charis Wilson when she was barely out of high school . Charis' indefatigable support was clearly a key element in the arc of Weston's culminating accomplishment. Of course when Charis demanded some public recognition of her creative contributions, that was more than Weston's ego could sustain and so she had to go too.