I went to the first chapter of every book and downloaded the html source. Then I ran a little shell script that isolated the lines that contained "Chapter_Combo.Add", then inserted the pageid listed on that line into a URL. They appear to all be sequential except for Bereshit that skips a chunk in the middle. The lines look like this:
Chapter_Combo.AddItem("Chapter 1", "15785");
Chapter_Combo.AddItem("Chapter 2", "15786");