Music Hall finished at the start of World War 1 in commecial thereafter becoming vaudeville or variety. J B Priestley wrote an excellent work of fiction called Lost Empires

There is A British Music Hall Society which meets regularly and there is plenty of material also there is the Concert Artistes Association as well which covers this area

There are lots of compilations of MH songs such Cockney DingDong produced and illustrated by Charles Keeping

I worked as a singer on the last so called MH at Ally Pally with Horace Mashford who did the full chairman act and also at the Battersea Town Hall These were paid gigs with professional acts. During the 80s MH and variety acts could scrape a living doing Old Peoples Homes and bingo nights doing "turns"

Re Gracie Fields - she was NOT Music Hall and was not popular during the war as she left the UK to its devices and lived overseas