The IHAA rules are all accessible here (click on the headings to go those rules) :
Major championships will run the rules listed under Horseback Archery Eventing.
The IHAA Standard Rules (with the 10m/s rule)
Korean events (K123, K23, K125, K12, K13)
(there are no additional Hungarian tracks - just the -100 and -90 covered within the Combined rules)
In case of minor adjustments or corrections there may be an annex published on the rules page of the IHAA website; this can be printed out and kept as an addendum to rulebooks.
* Addendum to the rulebook - clarifications and additions (updated 27th June '18)
As English is the working language of the IHAA, in case of confusion arising from translation of the rules, the English version contained in the printed rulebook (sections 1-3) and copied here here should be referred to as the Official Rules.
The course database, with information on a much wider variety of tracks, written and submitted by others