Lisa Cheney does quote some interesting material unique to her book, but I can't say that I value her opinions -- unlike W A Darlington's 1938 "J M Barrie" which I think superb, particularly on Barrie as a dramatist (Darlington was theatre critic on the Telegraph for 50+ years)!
Mackail's 1941 biography is "required reading" for any serious Barrie student, albeit a little daunting at times. It also suffers from a style that I personally find rather irritating, but is nevertheless a tour de force, and pretty much indispensable in terms of a blow-by-blow, day-by-day account of JMB's life. Mackail also knew Barrie personally (he was in his Allahakbarrie cricket team for a while) and was an intimate contemporary of the Llewelyn Davies boys (I rather think Jack had an affair with his sister!)
Roger Lancelyn Green's "Fifty Years of Peter Pan" is likewise required reading, and a good deal more enjoyable - he also wrote a slim monograph called simply "J M Barrie") which is excellent - and succinct to boot!
Janet Dunbar is particularly good on Cynthia Asquith... speaking of whom, her "Portrait of Barrie" is fascinating, whatever one's feelings about her role in Barrie's life.
J A Hammerton is the sole source for much of what we know about Barrie's early life, although his style is a little too hagiographic for my taste.
Piers Dudgeon is worthless.