You can access all the game manuals that we've created for Crusader Kings II, including expansions, on the CK2 Wiki here!
Unfortunately we don't have manuals for all expansions, but those that exist can all be found under the link above. We aim to release manuals in English, Spanish, French and German.
The wiki in itself is also functioning as a manual in itself, and you can find information on most things in there. We also appreciate all the content being created by the community for our wiki. So if you have knowledge that is not found on the wiki, feel free to add it and receive the gratitude from Paradox as well as your fellow players.