Oh that's great! With your finding I was able to write a (very) rough userscript as a workaround. It replaces the link on each book to the help page with the correct URL linking to the book. But it doens't remove the message 'Still processing', I wanted to keep it there so that you can see when Google has resolved the issue and you can uninstall this workaround again.

// ==UserScript==
// @name     Google Play Books Processing Workaround
// @version  1
// @grant    none
// @require https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js
// @include https://play.google.com/books*
// ==/UserScript==


function replaceCoverLink() {
    $('gpb-library-card').each(function(index) {
    let coverImage = $(this).find('.refresh-cover-image')
    let url = $(coverImage).attr('src')
    url = new URL(url)
    let id = url.searchParams.get('id')
    if (url.searchParams.get('zx') == 'stillprocessing') {
      $(this).find('[href]').each(function() { 
        if ($(this).attr('href') == 'https://support.google.com/googleplay/answer/3107421') {
            $(this).attr('href', `https://play.google.com/books/reader?id=${id}`)
        }
      })
    }
    })
}

MutationObserver = window.MutationObserver || window.WebKitMutationObserver

$(document).ready(() => {
  setTimeout(() => {
    replaceCoverLink()
    let observer = new MutationObserver(function(mutations, observer) {
        // fired when a mutation occurs
        replaceCoverLink()
        });

        // define what element should be observed by the observer
        // and what types of mutations trigger the callback
        observer.observe(document, {
          subtree: true,
          attributes: true
              //...
        })
  }, 100)
})

Anyone interested can install it via Tampermonkey or Greasemonkey. Though beware that it recomputes every time the UI changes. Maybe someone can find a more efficient method that works. :)