The elements of fiction - and what makes a nonfiction novel

I've been making some notes about plot and fiction, and the book I'm reading now provokes some further thought: Jeanette Walls's "Half Broke Horses" is touted as a "true life novel" - which is exactly what? I guess fiction, or novels, exist on some kind of continuum, in that all of fiction uses the same elemental material: experience, imagination, knowledge, and emotions. To a degree, all novels are "true life" though they vary in the extent to which the novelist/author uses his or her experience and the extent to which the author uses knowledge. Historical fiction is based mostly on knowledge, but not entirely - to make a story about a past era come "alive" the author draws on personal experience such as the way soldiers might talk and behave, the look of a city or a battlefield that they may have visited, and so forth. Norman Mailer, most famously, reversed the equation, writing what he called a "nonfiction novel" about Gary Gilmore's crimes and his execution: the facts were all something Mailer researched and learned, but the novel was full of Gilmore's imagined interior life, Gilmore was as much of a literary character as any other figure in any other novel - yet the novel carried an added frisson because we knew the events were real. Walls's novel "true life novel" might better be called a "fictive memoir": she's writing about a real figure, her grandmother apparently, so the facts are events are not at all familiar to us - but part of our pleasure in the book is our sense that these things - the hard life for a young girl on a ranch in Texas and New Mexico in the early 20th century, her early forays as a schoolteacher in Arizona and later as a factory worker in Chicago - really happened. Wallis, apparently, drew on family lore, memories, maybe documents to create for her grandmother the memoir that her grandmother might have written. It's entirely credible and very well written, and if you like these kinds of memoirs, and there are many, you will love this book - but don't think of it as a novel because it isn't, in my view. The author's use of the imagination does not make a book a novel: this book has no plot per se, no arc of a story, no development of character other than through the concatenation of events that constitute the narrator's life story. Will remind you of other find writing about women in the American West, notably Willa Cather.