Greasy Fork is available in English.
Saves all available Preview pages from a Google Book as PNGs
< Feedback on GBookDown - Download Google Books
The problem is in GM_xmlhttpRequest that doesn't fire any event.
Also, it lead to an looping in script.
Doesn't work with Greasemonkey 4+Firefox 57/58. Works with Greasemonkey 3+Firefox 56
When you press start on Greasemonkey 4+Firefox 57/58 it detects the number of pages but just sits there at "I already have 0."
Downloading Firefox 56 portable, installing Greasemonkey 3.17, then installing this script works great.