The QuestWorlds System Reference Document (QuestWorlds SRD) contains all the rules. However, it is not a rulebook, but rather a summary of all the rules for other publishers (including fan projects such as Humakt e.V.). The QuestWorlds Core Rulebook will be published by Chaosium Inc. on their website: https://www.chaosium.com and elsewhere. We highly recommend that you purchase the Core Rulebook as soon as it becomes available.
Previous versions of the QuestWorlds SRD used their own Open Gaming License. Recent versions use the Open RPG Creative License (ORC License).
A PDF copy of the QuestWorlds SRD can be found here (version 0.97).