It has been a long time since I have had to dive into a large code base myself. But in the recent couple of years I tried many times to get new developers into teams where we had an existing, rather large, code base.
And the method that we have used successfully, and I would say is the most effective way without question IMHO, is pair programming.
In the last 12 months, we have had 4 new members to the team, and every time, the new member would pair with another member well acquainted with the code base. In the beginning, the older team member would have the keyboard. After about 30 minutes we would pass the keyboard to the new member, who would work under the guidance of the older team member.
This process has proven to be quite successful.