From the Godfather of American Wargaming, Jack Scruby, comes the Rules For A 30 Years Wargame. We have the people at Historifigs to thank for offering these free reprints of classic wargames rules.
Tilly’s Very Bad Day is a set of free wargames rules for the Thirty Years War. They are designed as a fast play set.
This site has a good history of the Thirty Years War, with lots of of information about the various phases of the war — The Bohemian Rebellion, the Palatinate, etc.. Lots here that I never knew about.
If you’re interested in gaming the ThirtyYears War, there’s a post here about a set of rules for the period.
Tercios Brevis Editio is a free version of the Tercios rules for the Thirty Years War.