The summer days of our youth can feel like a sun-bathed path of endless possibilities. Ten-year-old Ethan has a lot that he’s looking forward to, but one night in July changed everything for him, his family and his neighborhood.

The approach of summer also brings a new novel by Riley Sager, the best-selling author known for his thrillers, “Middle of the Night” (Dutton, 352 pp., ★★★ out of four) out now.

Ethan Marsh is back in Hemlock Circle, the quiet fictional New Jersey neighborhood where he grew up, and it hasn’t changed much since he was last here. Almost all the same neighbors remain, too, except for the family of Billy Barringer.

Billy was Ethan’s best friend and next-door neighbor, but one summer night in 1994, Billy disappeared from Ethan’s backyard while the boys were having a sleepover in Ethan’s tent, and he was never found or seen again.

And now 30 years later, Ethan has reluctantly returned, haunted by his memories… and maybe something else?

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It wouldn’t be a Riley Sager novel if weird stuff didn’t start happening pretty much right away.

Ethan’s not sure if the mysterious occurrences behind his parents’ house or around the neighborhood are real, a cruel prank or just a figment of his sleep-deprived imagination, but the increasing number of eerie events can no longer be ignored, so Ethan starts his own investigation into what’s happening now, and what might have happened to Billy all those years ago.

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Sager’s novel, one of his first to focus primarily on a male protagonist, doesn’t linger with Ethan — or even in the present — jumping between now and the ’90s, peppering Ethan’s investigation with the events leading up to that fateful July night seen through the eyes of preteen Ethan, Billy, Ethan and Billy’s mothers and other assorted kids from the neighborhood.

Several of those kids, now adults, haven’t strayed far from Hemlock Circle, reconnecting with Ethan in his truth-seeking journey. There’s Russ next door, a family man and very different from the short-tempered kid that used to tag along with Ethan and Billy; Ethan’s old babysitter Ashley, who is now a single mom to super-smart, sweet Henry; and Ragesh Patel, former neighborhood bully who is now a no-nonsense police officer.

In typical Sager style, there are many sudden turns as the story builds, quite a few suburban secrets to uncover and there are so many questions: what happened to Billy? What’s happening to Ethan? What was really happening behind closed doors on Hemlock Circle? Is Hemlock Circle haunted by ghosts?

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But even as the truths untangle and reveal themselves in Sager’s novel, many of the deeper questions about Ethan, his relationships and the losses from which he never really moved on will largely go unanswered here. Disappointing, but perhaps realistic as an exploration of trauma.

Grief can be complicated, and can affect everyone differently. But it can’t be ignored, the body knows.

Sager’s “Middle of the Night” is a twisty mystery with a touch of the supernatural, but it’s also about the complexities of friendship, those fleeting but overwhelming feelings from growing up and coming to terms with profound grief.