In addition to Alban's information on the Freescale material there are also at least 5 or 6 books on the subject in different languages. A book on the HC12 will cover the most important information on the S12, but if can get an edition that covers the S12 then that will be better. See your local book supplier for the titles that are available.
There are also professional training courses from various companies.
Finally, the MCU is widely used in University courses and various of these are available at least partially on-line