Family. It’s humanity’s foundational social unit, one that encompasses myriad rich and complex dealings, which makes it somewhat surprising that more musicians haven’t devoted more attention to examining the myriad interpersonal dynamics that they engender.

But then there’s Loudon Wainwright III, who in nearly 50 years of writing songs and recording albums has built a deep body of work rooting through the various facets of family.

He explores that favored topic in even greater detail in his first book, the memoir “Liner Notes: On Parents & Children, Exes & Excess, Death & Decay, & a Few of My Other Favorite Things” (Blue Rider Press, $27).

A stroll through his estimable catalog quickly reveals where the major focus of his attention over the decades has been, with such titles as “Your Mother and I,” “Unhappy Anniversary,” “Your Father’s Car,” “Be Careful There’s a Baby in the House,” “Bein’ a Dad,” “Father-Daughter Dialogue,” “I Knew Your Mother,” “Dilated to Meet You,” “Older Than My Old Man” and “Surviving Twin.”

“It’s what I’ve been doing my whole career,” Wainwright said recently by phone from his apartment on Manhattan’s Upper West Side. He’s gearing up for a run of book signings and concerts that will bring him to L.A. for a Q&A with his friend, actor-director Christopher Guest on Friday at Largo, and back-to-back solo performances Saturday at McCabe’s in Santa Monica. “I’m used to doing that, writing about the people in my family, the women in my life.”

The real-world result has often been strained, or broken, relationships, but Wainwright has wrung out of those troubled situations a treasure trove of songs sometimes stabbingly funny, sometimes painfully revealing, but always brutally honest.

“His tone has been a giant influence on me,” said comedian and filmmaker Judd Apatow, a longtime Wainwright fan who has used him as an actor and musician in movies, including “Knocked Up” and “The 40-Year-Old Virgin” and in his short-lived TV series “Undeclared.”

“He used to appear on David Letterman’s morning show (in 1980),” Apatow said. “I remember being 12 or 13 years old and watching him sing the song ‘Unrequited (To the Nth Degree)’ (on the show). It was a hilarious, bitter breakup song.”

At this point, Apatow recites the song’s entire opening verse by heart: “ ‘When I die/And it won’t be long/Hey you’re gonna be sorry/That you treated me wrong/Yeah you’re gonna be sorry/That you treated me bad/Hey and if there’s an afterlife/I’ll gloat and I’ll be glad.’

“I found it so funny, but also very emotional and real at the same time,” he said. “I am such an admirer of his artistry. He’s a constant touchstone of what I should be shooting for; he’s the bar of sticking to your own high standards and to continually challenging yourself.”

The jacket for “Liner Notes” also features a back-cover testimonial from no less than author Salman Rushdie, lauding Wainwright as “a true original; not like anyone else, just as he set out to be.”

The book, Wainwright notes, essentially emerged from the experience of working on “Surviving Twin,” his one-man play that took its title from the song, both of which focus on his complicated relationship with his father, Loudon Wainwright Jr., the respected longtime Life magazine columnist who died in 1988.

“My editor at Blue Rider, Peter Gerthers, saw the ‘Surviving Twin’ show during an incarnation a couple of years ago in New York,” Wainwright said. “It spurred him to suggest that I could write a book or that he thought I had a book in me.

“Then somebody offered me some money to do it,” he said with one of the laughs that regular punctuate any conversation with him, “and so I was up against the wall.”

In “Liner Notes,” he addresses many of the themes he raised in the show — his combination of admiration and jealousy toward his father’s fame and writing talent — and includes several of his father’s Life columns to illustrate various points he’s making.

The life reflections expand from there, and he touches on his rocky first marriage to the late Canadian singer-songwriter Kate McGarrigle, of the McGarrigles, who is the mother of their equally celebrated children, singer-songwriters Rufus and Martha Wainwright.

In a passage in the book’s chapter that is devoted to McGarrigle, Wainwright recounts their wedding night with brutal honesty, and a sharp sense of humor: “I spent most of my wedding night in front of the tape recorder, drinking beer and listening to mixes of my soon-to-be-released second album, while my bride softly cried herself to sleep in the bedroom down the hall. It was a fraught and inauspicious beginning for the marriage, and I’m sure you’re thinking, What a selfish bastard. But wait a minute, it gets worse.” He proceeds to detail just how much further downhill things went from there.

The book also references his romantic relationship with Suzzy Roche of the folk-pop trio the Roches. She gave birth to their daughter, Lucy Wainwright Roche, who also has demonstrated considerable musical chops as a singer-songwriter. His second marriage, to actress Ritamarie Kelly, produced daughter Lexie Kelly Wainwright, yet another budding singer and songwriter.

“I wanted to lay (my life) out as it was,” he says now, “but I didn’t want to lay into anybody in particular. It’s not a tell-all book about other people. I copped to stuff I’ve been copping to in the songs for 45 years: drinking, womanizing. … I’m used to doing that.”

Yet he also writes and sings about other subjects. In fact, outside the cult of devotees who’ve lapped up each of nearly two dozen studio albums since his 1970 debut — which landed him among a string of literate-minded singer-songwriters dubbed “a new Bob Dylan” at the time — he’s still best known for his one and only Top 20 billboard hit single, “Dead Skunk,” from 1973.

He won the 2009 Grammy Award for traditional folk album for “High Wide & Handsome: The Charlie Poole Project,” his tribute to the early 20th century banjo-laying troubadour.

“I’m always drawn to people whose work seems vital and keeps digging deeper,” Apatow said of Wainwright. “They make you realize you can be as vital as long as you want, if you have the interest and the energy to do it.”