It took me awhile to come around on this -- two books in one about female friendships, art, anarchy, and trolleys, among other things -- but by the last pages, I didn't want it to end. The Book Borrower is odd and a little slow moving, yet the characters are fascinating and well drawn.
Berry Cooper (or Gussie/Jessie Lipkin) in particular is captivating. Cooper is a sculptress, a centenarian, who was the subject of a book (of the titular book borrowing) that her sister wrote about their youth, when Cooper was suspected of causing a trolley accident as an act of political protest. By chance, Toby Reuben and her family are drawn into Cooper's orbit, not realizing, at first, that she is the subject of The Trolley Girl, passed among friends. Cooper's still a firebrand, a woman who understood "that feeling bad is sometimes necessary." Though she's unreliable, and you often wonder whether she's touched with dementia, she presents an interesting vision of a woman who lives completely for herself and her art, unconcerned with following a well-trod path---indeed, Cooper holds, "People who think evil but unpredictable things are not as bad as people with predicable minds."
It'll be interesting to contrast this more closely with Claire Messud's The Woman Upstairs, which touches on some of the same themes.