The Official Handbook of the Marvel Universe (and its immediate successor, the Official Handbook of the Marvel Universe Deluxe Edition) was a perhaps unlikely success story in comic book publishing. It was a series describing the powers, histories and attributes of every major character (and some minor ones) in the Marvel Universe. The original version was released starting in 1982, ran for 15 issues, and was almost immediately succeeded by the deluxe edition, which ran for several more years, and was then almost immediately followed by an update series. The books barely seemed like comic books: they were at least half text by space and fairly dense text at that.
Although Marvel had always tried to present a more unified universe and narrative than comic books had traditionally done, they were also trying, like most commercial purveyors of serialized entertainment, to make the entertainment accessible to people who weren't hardcore fans. With that in mind, continuity was often bent and powers and attitudes of heroes and villains were altered for the story of the moment. Dialogue and exposition was also kept to a level that would be understandable to a younger reader. But in the early 1980s, such things were changing. There was a direct market for dedicated comic book fans, who were a bit older than they were before. A random eight year old kid walking into a drug store probably wouldn't pick up and buy something over a dollar (in the money of the time) filled with thick text. An older fan, a dedicated Marvel enthusiast, walking into a specialty store, would and did.
The production values for the writing and editing on the guides was very high. Mark Gruenwald, the editor (and Marvel's oracle of continuity) took his job very seriously. Each entry showed a picture of the character in question, as well as other panels depicting them in action, and the vital statistics, history and powers of each character. The covers of the guides showed a continuous mural of the characters. The writing, as I said, is very dense. I remember the writing as being very heavy on vocabulary for my elementary school self, and upon rereading it today, I find it still is. The science and technology is described consistently, although it is obviously full of fudge and pseudoscience. As a third grade student, I learned a lot of my basic scientific terminology (like biochemistry) from the Handbook.
Another aspect of the book that was influential on me, and perhaps on other readers, was the immense amount of hyperlinking put into the Handbook. Marvel had always tried to interlock its universe, in an attempt to get fans of one series or character to read other Marvel titles. The Handbook was full of parenthetical references to other entries in the Handbook, which showed the deep interconnections in Marvel continuity. When I had a sufficient amount of the volumes, I played a game where I would start following those references and see where they would take me, circling through the entire book. I think that this was my first exposure to the type of hyperlinking madness that would later make the internet (and this site) especially intoxicating for me.
The three handbook series were a first sign of how seriously the new comic book fans took comic books, and it is good that Marvel was willing to take their fans desire for information so seriously.