Run in google chrome -> inspect element -> source -> snippets at https://play.google.com/books to download all books on page as EPUB rather than doing it manually one at a time.

download-epub.js

function sleep(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}

let dots = document.querySelectorAll("[aria-label=\"More Options\"]")

for (let button of dots) {
    button.click()

    let exp = document.getElementById("overflow-export-ebook")
    exp.click()

    await sleep(1000);
    let epub = document.querySelector("[cdkfocusinitial]")
    epub.click()
    await sleep(1000);
}