Marc Robine's Grand Jacques : Le Roman de jacques Brel is close to 700 pages, including interviews, discographies, etc. I've not yet read any other works on Brel, although I think I can firmly say that this exhaustive (and wonderfully exhausting) work is among the (if not the) best of all time.
This is no hagiography, and I'm sure Brel himself wouldn't have welcomed one: he was mildly homophobic for the time (David Bowie being dubbed as a 'queer'), more than mildly (I'm not too sure, as he was a pop god of a long gone era, so kind of untouchable) phallocratic in that he wasn't content with one woman, but on the plus side I didn't detect a single sniff of racism about him. As for champion of the underclasses?
Depends on what is meant by underclasses. Brel was generous with money and time, a kind of hero, and we have only to listen to his songs, performed with such showmanship, such theatricality, to understand his sympathy for outsiders, shy people, those stricken by alcoholism, by age or unrequited love, to realise that Jacques Brel was a master of many arts he attempted: singing, acting, writing, etc. This book is an immense tribute to the huge talent that was Brel. Chapeau bas, as Barbara would certainly have said.