RaggieRags: The download is now called "cluebook" (used to be "cluebooks"), so I take it there's not going to be a Serpent Isle cluebook.
I really don't appreciate this. I bought the whole damn package because of the Serpent Isle cluebook (which I've never personally seen sold for 10 dollars on eBay) and now they have retroactively changed what's included in the package. I e-mailed them and asked to return my money if the cluebook is not included, and the answer was that it's going to be included next week. That was back when this game was released on GOG.
I agree that the GOG package should include all the clue books - after all, Serpent Isle's book is one of the easiest to find and they even got Ultima III clue book, which is quite rare and expensive.
At the moment there's at least one SI clue book on sale on eBay for $19,99 (about 15€) + postage. I don't think that's too expensive, but maybe that's just me.
However, if you're only interested in good quality scan (PDF) of the clue book you can download one from elsewhere, for example here:
http://originadventures.wordpress.com/category/books/serpent-isle-cluebook/
All in all, it's still quite a bargain to get Ultima VII and Serpent Isle plus the add-ons for $5,99 even if they don't include the missing clue book later.