SFGate: “Learning from the multivolume sagaists who have preceded her, she shows much skill in writing those enormous histories into this narrative about two grown women choosing the paths their adult lives should take.”

It is only proper that as Elena Ferrante’s Neapolitan series has grown, each new novel has engulfed its antecedents, absorbing and assimilating them into its basic fiber. When “My Brilliant Friend” leaped into America in 2012, it sparked a cult sensation with a twinned coming-of-age story of two girlhoods. The arrival of “The Story of a New Name” the following year placed those girlhoods into a story of adolescence yearning toward adulthood. And with the appearance of “Those Who Leave and Those Who Stay” last year, girlhood and adolescence were leveraged into a chronicle of young adults.

Now, in the final piece, “The Story of the Lost Child,” Ferrante has two mature lives to work with: weddings, divorces, children, affairs, bitter rivalries, successes and failures, regrets, grudges, fears and a few remaining hopes. Learning from the multivolume sagaists who have preceded her, she shows much skill in writing those enormous histories into this narrative about two grown women choosing the paths their adult lives should take.

As Book 4 picks up, our narrator finds herself deeper in uncertainty than ever. Lenú has left her brilliant professor husband, Pietro, for her childhood crush, Nino (himself embarking on a promising career as a politician), but is the seductive technocrat trustworthy? Likewise, her life as a novelist is under way, but she is finding it impossible to get started on her next book, whose advance she has long since spent. Should she remain in grimy, crime-ridden Naples or flee? Should she let herself grow close again to Lila, or avoid the woman who has held a frightening power over her since childhood?

Unanswered questions have always been the motor propelling Ferrante’s massive opus forward, and “The Story of the Lost Child” has more than ever. So many that at times it feels as though every single one of this book’s 150-plus subsections ends on a miniature cliffhanger, egging readers toward answers that Ferrante tends to explore with a skill worthy of her gifts.

However, the firm stuff of this narrative is occasionally softened by subplots that feel like mere contrivance. When halfway through Book 4, Alfonso, the brother of Lila’s first husband, begins dressing like Lila (and looking as beautiful as she), this shocking development comes out of nowhere. The eventual end to Alfonso’s story — murdered as a drugged-out homosexual — is just as desultory, lacking in the imagination that have made the Neapolitan novels so good.

The biggest of Lenú’s unanswered questions is, of course, Lila, and here Ferrante has an enigma worthy of endless contemplation. When Book 4 starts, the two women are on the outs, but, racked by guilt at having abandoned her family and her past, and hoping to kindle literary inspiration, Lenú returns to brutish Naples, letting Lila creep back into her life. This gives Ferrante the opportunity to return to the immensely complex, shifting question of the women’s friendship; by contrast, the other characters and plotlines in “The Story of the Lost Child” merely furnish this feast.

As always, the central question is why Lenú cannot help but be fascinated and overpowered by Lila. As Ferrante’s narrator matures into a feminist writer of increasing renown, she is nagged by the suspicion that Lila is in fact smarter: It is really her friend’s brilliance that animates her pen, and, even worse, Lila may be working on a massive history of Naples that will put her novels to shame. Despite Lila’s uncultured life — hardly reading, never leaving the old neighborhood and remaining as trapped as ever — she seems to have crucial insights that Lenú lacks.

Part of this insecurity derives from Lenú’s inability to choose her domain. A successful writer who has married one powerful intellectual and wooed another, she nevertheless remains a failure in her mother’s eyes, a disgrace to her roots. As ever, Naples is both a trap and a source of identity. Where Ferrante previously explained how Lenú fled, she now draws back the layers toward understanding why she eschews safer, more comfortable alternatives, choosing to imperil herself and her children with a harsh life. This fortitude and independence are what ultimately make Lenú original and sympathetic. She is a woman possessed of resolute will, but also fallible for reasons not even known to herself, complex and honest enough to fascinate after well over a thousand pages.

That thing we all want most — to know ourselves — may become possible only when the questions set in place by childhood have been resolved. By remaining in Naples, and therefore in constant contact with their families, adversaries, memories and one another, Lenú and Lila choose to remain close to those original dilemmas. The ones they resolve — and those that continue to play out until the very last pages of Book 4 — show what is at the heart of these two personalities.

Ferrante concludes Lenú’s saga with an ecumenical 100 pages called “Old Age: The Story of Bad Blood” — the elemental, dour finality of that title gives a good sense of where our novelist leaves this story. The order that has defined the women’s neighborhood slowly dissolves, as do the lives they have spent their youths building. The world around them moves on, but they remain battling through the same relationships that have guided their lives. So many facts of Lenú’s existence have become manifest, but that doesn’t mean that any of us will be permitted the truths we most want to know.