Stéphane Fidanza (sfidanza) created a series of printable charts for Age of Heroes that you can use while playing the game (without alt-tabbing). They cover the Heroes of Might and Magic V skills, creatures and abilities, both in English and French. Check out this page to download them.

Speaking of things that should have been in the manual in the first place, I've heard rumors about a fan-made user manual project in Germany. Let's hope it will become a reality.