Henry Bushkin’s “Johnny Carson” is that rare celebrity tell-all by an author who knows whom and what he’s talking about. Though early readers have been shocked, just shocked, by Mr. Bushkin’s treachery, they have also been drawn ravenously to his book.

Mr. Bushkin was a young lawyer in 1970 when Carson, quite inexplicably, decided to become his client. He was a sadder but wiser one by 1988, when Carson abruptly fired him and then went to war, accusing him of negligence, malpractice and other improprieties. But between those temporal bookends, this lawyer and his star client shared a lot of time and a complex bromance.

Still, Mr. Bushkin did not regard himself as very important to Carson, so he was surprised to learn from Kenneth Tynan’s 1978 New Yorker profile of Carson that Carson thought Mr. Bushkin was his best friend.

It’s easy to approach this book thinking that its author has an ax to grind. Maybe he does, but his account sounds unexaggerated, credible and willing to place blame wherever it belongs. Mr. Bushkin wonders now at his own naïveté when Carson instantly adopted him as a constant companion. But he was recruited when Carson was ending the second of his four marriages, and a malleable new lawyer would be handy.