Apparently, after I read the big green book, there was a reprinting that broke it up into 3 volumes. I thought they were sequels.The first book? There's only one book and it's a doorstopper. He did write a series in which aliens attempt to curb or stop global warming or something like that.
Some of the world-building in Battlefield Earth is interesting, for example, some of the various aliens have a different number base other than 10.