This will seem a bit silly, but one thing I loved as a pre-teen was not necessarily READING the Official Handbook to the Marvel Universe, but knowing that it existed. I really enjoyed the fact that every little character that showed up in Marvel Comics had an entry that talked about their history, origin, first comic appearance, strength level, hair color, base of operations, other aliases, and whether people know his or her secret identity.
Having that attention to detail in a world that is entirely in people's minds was a fantastic thing to me, and it made it seem, if not real, then at least consistent (which it probably wasn't, entirely). If you're going to escape into another universe, it might as well be a cool, well-populated one, right?
But possibly the best part of the whole thing was the covers of the deluxe edition of the series. Click on the thumbnail below to see the image (Warning-- It is 2.88 MB.)
Note in particular Mr. Fantastic's arm which spreads across something like ten issues, and the fact that the last cover leads right into the first again. Pretty cool for me as a teenager.