LOS ANGELES — Billy Crystal has a side to him that few get to see. When he isn’t being genial or eager to please, he can go to a place that his friend Robin Williams calls “the other side.” There, he is free to be funny in a crankier, more caustic way, and he becomes — in Williams’ words — “an angry old Jew.”

Among the topics that can bring this out in Crystal, 65, the comic actor, director and perennial Academy Awards show host, are the acknowledgment of his advancing years; personal losses he has experienced; and, tangentially, the labored creation of his 2012 film, “Parental Guidance,” which cast him and Bette Midler as unconventional grandparents, and which he said was not an easy sell at the major movie studios.

“When you get to a certain age, they — hmmm — hesitate a bit,” he said last month in his Beverly Hills office. “They’re not sure there’s an audience. I said, ‘There’s 77 million people wanting a story for them.’”

Crystal added that he understood if Hollywood no longer saw him as the diminutive if dashing star of romantic comedies like “When Harry Met Sally …”

“It’s not easy to go through that when you can’t get the girl anymore,” he said, unfurling a mischievous smile. “You can, but usually you both die.”

Crystal’s delicate relationship with old age — if it were a person, he would want to embrace it and throttle it — is a subject that infuses his new book, “Still Foolin’ ’Em: Where I’ve Been, Where I’m Going, and Where the Hell Are My Keys?,” which Henry Holt will release Tuesday.

Part memoir

The book is partly a memoir in which Crystal recounts his growth from a Long Island youth into a husband, father and grandfather, and his professional evolution from struggling stand-up to seasoned star.

It is also punctuated with freewheeling chapters in which the author riffs on the ways age has affected him and his generational cohort, causing beloved body parts to wither and droop, and taking tolls on their sex lives.

In one section, Crystal imagines a series of exchanges between a 25-year-old couple in 1973, and that same couple, now 65, in 2013. For example, in 1973, “Her: Let’s stay home and make love. Him: I love rainy days.” And, in 2013, “Her: Let’s make love … Him: My hip hurts, it’s going to rain.” The book offers numerous reminders of how Crystal has spent his career capably serving multiple contingencies: young and old, celebrity pals and the mensch on the street.

Yet, as he takes stock of his legacy, it is also an occasion for him and his peers to look at the amiable, all-around entertainer that he is and wonder if the sun is setting on that style of showmanship.

“Sometimes, it does feel like we’re down to the end of a species,” Williams said in a telephone interview. “When the last female dies, that’s it. ‘I guess I won’t have to sit on these eggs anymore. Good luck and thank you.’”

Sitting in a rocking chair and surrounded by career mementos — a dynamic photograph of him riding a horse in “City Slickers”; the uniform he wore for an at-bat with the Yankees — Crystal said he was not necessarily burdened by all the inconveniences and afflictions he writes about in “Still Foolin’ ’Em.”

“I’m in really good shape,” he said. “It is a humor book.”

But Crystal, who in person is more low-key and contemplative than his stage persona might suggest, knows as well as anyone that he has lost a few hairs and gained a few forehead lines since the first of his nine Oscars gigs in 1990. Opening up about this to members of an audience that has grown up with him, he said, is just another way of connecting to them.

At 65, he said: “I have more to talk about, and feel secure with myself enough to say, ‘I’m just like you.’ I found it very freeing. I’d read the book and go, ‘Should I? Yeah, why not.’”

Nostalgia

In a sense, Crystal has always been a professional nostalgist, with an admiration for comedy forebears like Alan King and Sid Caesar, and an act that included characters like Sammy Davis Jr. and the fictional Borscht Belter Buddy Young Jr. (“I was getting ready,” Crystal said. “Everything’s a rehearsal, man.”)

But he had more time to focus on the past, he said, during “a period where I didn’t make a movie for 10 years — a lot of it by my choice, and a lot of it by studios’ choice.”

More recently, Crystal said he has found himself riffing and writing on themes of aging and his personal life, occasionally trying lines out on talk show appearances and at social gatherings. Compiling these pieces into a book, he has introduced audiences to unknown parts of his life, and opened himself up to some unfamiliar experiences.

In June, Crystal gave a public reading from “Still Foolin’ ’Em” at New York University, his alma mater. His selections included a chapter called “Buying the Plot,” in which he and his wife, Janice, confront the reality of having to purchase their final resting places.

During this passage, Crystal became uncharacteristically choked up, and accidentally dropped the iPad he was reading from. He said later that it was because he had made the mistake of making eye contact with his wife, who was attending, while he read. But he did not mind sharing this vulnerability with the audience.

“It was pretty stunning that it took my breath away for a while,” he said. “But it’s real drama.”

Janice Crystal said in an email that her husband had consulted with her throughout the writing of “Still Foolin’ ’Em,” but “the more poignant chapters he kept to himself until he felt he had it just right.”

She said he had been open and expressive to her about his feelings, even if he didn’t take all of her editorial notes. (She asked him if he would remove a line from the book in which he compares his anatomy to “Einstein with Barry Scheck’s nose” because, she said, “I thought it might make Scheck’s kids cry.”)

The positive reaction to his readings has been heartening, Crystal said, when the responses could also have been, “Him again?”

Social media

Whatever detractors have said about Crystal over the years, he has probably thought it about himself at one time or another. While he tries to ignore his criticism, Web innovations like YouTube and Twitter have made it almost impossible for him to avoid it when he goes online and, he said, “when social media, really, is a pack of piranhas in the river of entertainment.”

In particular, the Oscars shows seem to generate feeding frenzies, whether Crystal participates in them or not. He is derided for hosting them almost too reliably and for being an overly familiar presence at them. Yet other emcees have struggled to reach the competing audiences — those attending the show and those watching at home — as easily as he has.

Crystal said he did not expect this tension to be resolved any time soon.

“They have so many awards that they still have to give, and the show is what it is,” he said. “You hope to get a good opening and a couple of spots, and you pray something happens. If nothing happens, it’s always going to be your fault.”

While he could not say for certain if he will host the Oscars again, Crystal said: “I don’t know if I need to do it anymore. I used to need to do it, because I loved doing it. At this point, there’s so many other things I’d like to get a chance to do.”

These days, Crystal is as busy as he has ever been. This fall, he will revive “700 Sundays,” his Tony Award-winning one-man show about his childhood, on Broadway. Then he begins work on an FX television pilot, “The Comedians,” which casts him and Josh Gad as performers who become reluctant partners on a sketch comedy series.

When roles have come less steadily, Crystal said: “All that time, you go, ‘God, am I slipping away here?’ And then something great happens, you get a call, and work begets more work.”

And if the comedy work should dry up completely, Crystal said he could always play over-the-hill horse trainers, like “the sweet spot that Mickey Rooney had in ‘The Black Stallion.’”

In the most cantankerous voice he could muster, Crystal said, by way of audition, “That’s the fastest quarter she’s ever run!”

Reverting to a more sincere tone, he added, “You don’t want to wait for that aged jockey role.”