Wow - just downloaded them and they are really nice clue books. Just wondering if there is any chance of getting one for VI?

I would like one as well, if it is possible. I could always find a guide somewhere online, but having one of such high quality as the GOG ones would be a plus.

They probably have the rights to make that book available; they probably just don't have a copy. They'll probably accept and use a donated copy; they're clearly willing to accept legally sellable-alongside-the-game materials from fans, as seen when they obtained development artwork and the manual for Earthworm Jim from Earthworm Jim fansite RocketWorm's webmaster Lemm.

AFAIK, there was never a Clue Book for MM6. Everybody was switching to Prima Strategy Guide and 3DO was not an exception. They even got horrendous M&M Compendium for all the previous games -- the strategy section for MM1&2 was a grand total of 2 (yes, two!) pages. I really miss old hint books...
The set of clue books on GOG is the same one published by 3DO on CD -- there are a lot of copies of it floating on EBay lately. Those are nicely processed PDFs but there are a few cosmetic glitches. MM3 clue book is missing a few non-informative pages (CD actually had that mentioned in the readme). MM1&2 covers are ugly black and white, the originals were actually in color and quite stylish.