The 12 volume 1970s Marvel Index series by George Olshevsky. What a grand starting point for learning about the vastness of the Marvel Universe
The various 1980s Official Indexes by George Olshevsky, which expanded the ideas of his original work.
The 1990s Official Indexes, which condensed a lot of data into a smaller package.
The Official Index series that ran from 2009 - 2012
The Official Handbook of the Marvel Universe A-Z, the ultimate 14-volume set of profiles covering thousands of characters, teams, places and events spanning Marvel's entire history, and subsequent updates and themed books that appeared afterward.
The Unofficial Appendix to the Marvel Universe is a constant source of information regarding obscure characters not yet covered by the Official Handbooks.
The Grand Comics Database feeds me all sorts of series details and publication information.
The Marvel Chronology Project gives me insight into the vast tapestry of character histories, putting their exploits into chronological order.