He adduces four serious — I’m trying to avoid saying “tectonic” — shifts that have taken place over the last 40 years. Combined, they make “This Town” read like the endgame chapters of Gibbon’s “Decline and Fall of the Roman Empire.” In addition to his reporting talents, Leibovich is a writer of excellent zest. At times, this book is laugh-out-loud (as well as weep-out-loud). He is an exuberant writer, even as his reporting leaves one reaching for the Xanax. As for those four big changes:

Lobbying. President Obama’s first year in office was the best year ever for the special interests industry, which earned $3.47 billion lobbying the federal government. Ka-ching — your change, sir. There’s a phrase in journalism-speak called “burying the lede,” which Leibo­vich appears to do by waiting until Page 330 to cite this arresting figure (previously reported by The Atlantic): in 1974, 3 percent of retiring members of Congress became lobbyists. “Now 50 percent of senators and 42 percent of congressmen do.” No one goes home anymore. Cincinnatus, call your office.

There are a number of sanctimonious standout “formers” in Leibovich’s Congressional hall of shame, but just to name a few exemplars who gleefully inhabit ethical no-worry zones and execute brisk 180-­degree switcheroos on any issue, including the Armenian genocide, so long as it pays: Dick Gephardt, Evan Bayh and Tim Pawlenty. (Christopher Dodd, late of Connecticut, is another beauty. Disclosure: he beat my uncle out of a Senate seat, but judge for yourself if he isn’t loathsome for other reasons.) My own modest proposal is that the media stop referring to these scoundrels as “strategic consultants” or their other camouflage titles and call them what they are: influence peddlers. I know — good luck with that.

The other major change took place pari passu with lobbying: the arrival of big money in Washington. “Over the last dozen years,” Leibovich writes, “corporate America (much of it Wall Street) has tripled the amount of money it has spent on lobbying and public affairs consulting in D.C.” Alongside this money comes the tsunami of dollars from presidential campaigns. He reports that during the 2012 contest, the so-called super PACs and megadonors pumped “upwards of $2 billion . . . into the empty-calorie economy of two men destroying each other.” He refers to a datum courtesy of The Huffington Post, which reported in the spring of 2012 that, so far, “the top 150 consulting companies had . . . grossed more than $465 million” during the campaign.

All of which has given rise to another unlovely development: political consultants and their concomitant celebrity. This breed has, Leibovich says, essentially replaced the old-style political bosses. One might ask: is it a bad thing that we now have the omnipresent James Carville and Mary Matalin and their ilk? Aren’t we better off for this “celebrity-industrial complex” instead of the smoke-filled rooms of yore? Over to you, but at least the boys in the smoke-filled rooms didn’t yap at us on TV on the Sabbath and endorse Maker’s Mark bourbon. (Honestly, James and Mary. They’re also doing the safety briefing voice-over for Independence Air. Is this a great country or what? Meanwhile, on “Good Morning America” tomorrow, George Stephanopoulos’s guests are. . . .)

Bringing us to the fourth change: Pandora’s (cable TV) box. The rise of cable television and the 24/7 news cycle, as well as Facebook, Twitter and the rest of social media, have provided all these people with heretofore unimaginable influence. “Suddenly,” Leibovich writes, “anyone without facial warts could call themselves a ‘strategist’ and get on TV. Or start an e-mail newsletter, Web site or, later, blog, Facebook page or Twitter following — in other words, become Famous for Washington.”

It has also enabled journalists to turn themselves into pundits, with all the glittery and greasy emoluments of that lower trade. “Punditry,” he writes, “has replaced reporting as journalism’s highest calling, accompanied by a mad dash of ‘self-branding,’ to borrow a term that had now fully infested the city: everyone now hellbent on branding themselves in the marketplace, like Cheetos (Russert was the local Coca-Cola). They gather, all the brands, at . . . self-­reverential festivals, like the April White House Correspondents’ Association dinner, whose buffet of ‘pre-parties’ and ‘after-parties’ now numbers more than two dozen — because a single banquet, it is clear, cannot properly celebrate the full achievements of the People Who Run Your Country.” Tom Brokaw, current wearer of the mantles of Walter Cronkite and Tim Russert, has now publicly declared he’s over and out and done with the damn thing, which has become a grotesque, narcissistic self-parody.