“Oh no,” I said to myself, noting the presence of another “new” book about Orson Welles. “Definitive,” I thought. Again. “The last word,” I was sure.  Again.

No other life in film has been so devoured, pored over and parsed. You could make a lovely flow chart of the ebb and flow of Wellesiana and the state of his legend — the nadir coming from Pauline Kael’s credit-removing “The Citizen Kane Book,” and Charles Higham’s “pathography.” Then there was all his former partner John Houseman wrote about Welles in Houseman’s own memoirs.

His reputation was revived Barbara Leaming’s “as told to” biography, Simon Callow’s two volume dissection, interview transcriptions from Welles acolytes Peter Bogdanovich and Henry Jaglom, “rethinking/setting the record straight” books by Frank Brady and Robert Carringer and Welles the Shakespearean (Michael Anderegg) and on and on.

But damned if Patrick McGilligan’s “Young Orson” doesn’t sum all those earlier works, open some new doors and close — with finality — several others.

In 750 or so pages, McGilligan thoroughly covers Welles’ pre-history – a family saga that might make for a great PBS series post-“Downton Abbey.” About 100 pages pass before Orson is born.

The last 50 pages cover the last day of Welles’ life.

In between, we discover “new” inspirations for “Rosebud,” new testimony (a 1950s copyright suit by the author of a biography of William Randolph Heart involving Welles, screenwriter Herman Mankiewicz  and Houseman) that settles several matters of authorship and biographical underpinnings for “Kane.”

And all that comes in the latter pages. McGilligan focuses on the fascinating debut Welles made, the run Welles had — reinventing the New York stage of the 1930s, conquering radio and frightening America with a take on H.G. Wells’ “The War of the Worlds,” and then reinventing the movies with “Kane.”

As many Welles books as I’ve read, I never tire of re-discovering his all-African-American “Macbeth” (glimpsed below), his transformative Fascist-era “Julius Caesar.”

He took his first shot at directing something like a movie as a teen — a silly “student” project, “Hearts of Age,” long available on Youtube. More recently, the footage he shot for a stage farce he was trying to turn into a multimedia (filmed intervals moving the plot forward) project surfaced, which showed him how important and how difficult editing was (he never finished it. See below).

And then there was radio, my first career (college, and just after), where I really fell in love with Welles, thanks to transcriptions of his “Mercury Theatre of the Air” broadcasts, and that playful “War of the Worlds.”

All leading up to the great climax of “Kane,” the fourth or fifth idea for a “first film” from the “boy genius” Welles.

“Young Orson” is a brisk read, illuminating — McGilligan uses a dogged pursuit of exact dates to tear apart some of the “myths” around Welles. No, he didn’t father a future filmmaker with Geraldine Fitzgerald. And he eviscerates and dismisses, for once and for all, the various labels slapped on the director by Houseman, Higham, Callow, Kael and others, including the mythmaker himself. Welles was something of a fabulist, you know. Couldn’t take anything he said about his history, his career and “Kane” seriously — without doing the research.

McGilligan did. A terrific book.