Hi fallen.
Just briefly... There is a difference between Old Guard & Old School. Old Guard is the old, mostly gay male leather culture that was at the front of the BDSM movement in the early 1970s through the 1990s. Old School is the actual training protocol & philosophy behind the Old Guard movement. There aren't a lot of "books" or "bibles" about what Old School is, because it is a traditional method of training & a lifestyle philosophy passed down from one Dominant to the next, one submissive to the next.
In a nutshell... Old School Dominants generally are brought into a "house" and taught. This usually occurs by being an apprentice to an older, more experienced Dominant, and "learning the ropes" so to speak. Many OS Dominants consider themselves to have at least some time "in service" as a submissive, often in the role of an apprentice Dominant. This apprenticeship is key to the Old School way, as it creates a continuous line of succession and passes on the traditions of Old School. It is akin, somewhat, to an oral religion/culture, whose traditions & history are passed down from one generation to the next. Additionally, submissives are taken under the wing of more experienced submissives, so that they can "learn the ropes" as well, and be guided in their journey. It is very similar to a "mentorship"-like program & works with the same general schematic.
As for what the OS philosophies & traditions are.... OS is grounded in protocols & respect. The main "pillars" of OS include discipline, service, respect, obedience & integrity. There are certain protocols, etc. that I can expand on, if you would like.
OS is 90% similar across the world---so if someone "professes" to be of OS training, you can generally quickly see if this is true... *grins*
Hope this helps some...
delia