Search for Vaudeville and Music Hall. They were the Victorian precursors of pop, with their own superstars, top-selling writers, and classic songs - now mostly forgotten.

There's quite a bit of Music Hall in McCartney's songs. (Lennon called it "That granny crap.")

One of the reasons copyright exists in the form it does is because many Victorian houses had pianos. Without recording technology there was a huge, incredibly profitable market for sheet music of light classics and music hall hits that could be played at home.

There's also a series of books by Simon Napier-Bell, who used to be a famous (notorious) pop manager in the 70s to 90s. Three of them are hilarious and frequently disturbing autobiographies and comments on the business.

The fourth is Ta-Ra-Ra-Boom-De-Ay, an almost academic account of the music business from the early 18th century.

There are some real surprises, and also a lot that's surprisingly familiar.

That’s a great question. I had to learn about it indirectly, gathering bits and pieces from novels of the time and offhand mentions in Beatles biographies.