Heirs to Merlin is the book of medieval England and Wales for Ars Magica.
This was my first full book. It is the source book for England and Wales, and contains no game statistics at all. Instead, I focused on ways to bring medieval history into a game, without turning it into a history lesson. I think that a large part of the fun of historical roleplaying, as opposed to pure fantasy roleplaying, is the feeling that you are actually taking part in history. On the other hand, the game should not feel like a history lecture, occasionally punctuated by a (roleplayed) fight. Thus, I concentrated on providing historical information in a form that could easily be incorporated into a game.
I still think that this is basically the way to go, and it is the approach taken in current Ars Magica books as well. If historical information can be made the basis of a player character, or central to a story, then you get the historical feel without sacrificing the fun. Getting people to be extremely careful about not mentioning anachronistic things tends to muffle the spontaneity.
However, there are some things I would do differently now. For a start, I would include game statistics, particularly things relating to character creation. More fundamentally, I don’t think there was enough medieval myth in the book; the balance was too far over to the historical side. You could use it as a sourcebook for an entirely non-magical medieval England game, and feel you had got your money’s worth. While that’s not entirely a bad thing, a book for Ars Magica should be a bit more mythic.