Books of advice come in many forms: financial, spiritual, physical, philosophical. Novels too are books of advice, if read in a certain light. Eve Babitz understood, for example, that part of Colette’s greatness is that you can open her novels anywhere and “brush up on what to do.”

There are only two advice books I’ve read more than once. One is Tom Hodgkinson’s “How to Be Idle” (2004). Its title is self-explanatory. The other is J.P. Donleavy’s “The Unexpurgated Code” (1975). Its title is less so. Donleavy’s book is a sendup of the form that happens to be, possibly, the funniest book ever written.

“The Unexpurgated Code” turns 50 this year. It has dropped from sight, yet here we are at a moment when the world could use it. It’s a book to turn to when you need a little pick-me-up. It is Bolivian marching powder for the spirit. Its table of contents alone is more happily anarchic than most books in their entireties. Here are a few of Donleavy’s 270 topics:

“Upon Placing the Blame for Venereal Infection,” “Upon Embellishing Your Background,” “Upon Being Unflatteringly Dressed in an Emergency,” “Upon Your Spit Landing on Another,” “Upon Fouling the Footpath,” “Upon Heaping Abuse on the High and Mighty,” “Upon Being Exorcised” and “Upon the Nearby Arrival of a Flying Saucer.”

Donleavy is best known as the author of “The Ginger Man,” his tumultuous 1955 comic novel about Sebastian Dangerfield, an American student living in Dublin. (Sample sentence: “All I want is one break which is not my neck.”) He is also the author of many other novels, plays and books of stories. His novel “A Fairy Tale of New York” (1973) inspired the title of the song by the Pogues and Kirsty MacColl that helps make Christmastime bearable.

Donleavy was born in New York City’s Brooklyn borough, to Irish immigrants, and grew up in the Bronx. He was the son of a firefighter. After serving in the Navy during World War II, he spent the rest of his life in Ireland. He was rarely photographed in anything other than layers of tweeds, so that he resembled a walking advertisement for 18-year-old Tullamore Dew.

Battered copies of “The Unexpurgated Code” pass among admirers like samizdat. The reason isn’t merely that it’s funny. The book clocks the absurdities of human conduct like few others. It takes note of the chutes and trapdoors and ladders and ejection seats involved in all human discourse. It says: We’re all miserable bipeds struggling for a bit of breathing room, so you might as well have a sense of humor about it all.

If you have been excluded from parties you wish you’d had the chance to boycott, if you lack long shanks, if you dine too often at low tables at bad addresses, if you feel as dented as a discarded pingpong ball, if you are not a member of the dividend-drawing classes, well, recall that Philip Larkin advised in a 1941 letter that “stupid ills need stupid remedies,” and turn to Donleavy.

A few weeks ago, in a restaurant, I was snubbed — in front of my family! I found out later that it was an entirely accidental snubbing, and all is well, but it stung at the time. When I got home that night, still smarting, I consulted Donleavy. Here is a bit of his advice in “Upon Being Snubbed,” which cheered me up instantly: “Take solace from the fact that it is unlikely that you will ever be kidnapped.” You will not find such counsel in Miss Manners.

You can flip to almost any page in “The Unexpurgated Code” and be reduced to helpless laughter. If you are not to the manner born and feel the need to defend your lineage, Donleavy writes, rummage around in your past: “Someone must have been something once.” He adds: “If you have received a Red Cross Life Saving Certificate, riposte pronto with this information.”

Donleavy covers a good deal of standard etiquette-book topics — how to behave at the table, the hair salon, the theater, the class reunion, the bank and while sick. (“Sneezing is one of the best ways of widely spreading your germs if this is what the people around you deserve.”) But it gets risqué.

There are also discourses on flatulence, notably as a method of communication between spies, on nose-picking and on the squeezing of pimples and blackheads. The latter maneuvers should be confined to people you know well, he writes, “although it is also one of the fastest ways to get to know someone better.” There are many strange chambers in this nautilus spiral of a book.

Some of the finest sections are on suicide, execution (“Relax and wait. Most things will be taken care of for you”) and death in general. He recommends that, if you learn you have but a short time to live, you “do not rush out to a night club or the latest celebrity joint and scare the hell out of everybody.”

Donleavy’s book is one for the world’s underdogs, its confirmed pullers of social boners, those who sense they are too often taking a worsting from reality. It might make you indescribably happy. Indeed, there is a section titled “Upon Encountering Happiness.” It reads, in full: “Be wary at such times because most of life’s blows fall then.”