For decades, the World War II generation was famously tightlipped about the wartime years. Only late in life have they finally been telling their stories.

But Tom Franks intended to keep his secrets to the grave.

He’d served in a mysterious Navy unit in some of the ugliest places in the Pacific and European theaters, and the dark memories he carried when he finally came home shadowed his life and that of his family.

That’s why his daughter, Pulitzer Prize-winning reporter Lucinda Franks, set out to know what he’d been through, what he was so determined to keep to himself.

“My father’s character had been a casualty of war. And the war he’d brought home had caused collateral damage: to his own daughters,” she writes in a complex, bittersweet memoir that mixes well-told history with a halting love story between a daughter and the father she describes as “the Man Who Wasn’t There.”

In “My Father’s Secret War” (Miramax Books, $24.95), Lucinda Franks uses relentless reporting and wily cajoling to break through his vow of silence at last. Along the way, she mines military archives and fast-fading memories – her father’s, as he sinks into dementia, and those of his aging comrades, whom she tracks down – to present an unvarnished picture of the personal sacrifice war can exact.

“Was my father a spy?” she asks after finding intriguing mementos stashed away as she helps him tidy up his apartment: a tiny camera, a gun silencer, disguises (including parts of a fake Nazi SS uniform). It turns out he was part of an elite force whose often code-named operations placed him, for example, in the assault waves on Pacific atolls and at the liberation of a horrific death camp in Germany.

But all her sleuthing barely seems to prepare her (or the reader) for his confession, “I was an assassin,” and the wrenching details that follow.

Although the book’s pacing occasionally lags and though readers may be disappointed that it does not reprint some of the treasured old photographs Franks describes, there is lovely writing throughout – “A bride’s veil of Queen Anne’s lace covered the face of the hill,” she writes at one point.

And this rich tale of war and family ends in a welcome, loving armistice between father and daughter.