A pocket sized guide is OK to carry, but the really important stuff in it you have to KNOW, you won't have time to dig the book out, find the page, then try to read the CPR (for example) instructions in the 50 mph winds that are blowing. By the time you get that done your victim is a gonner.

Re carrying oxygen, that stuff is the wonder drug of first aid. It is amazing the improvement of a victim, no matter what the ailment (with maybe a couple of exceptions), will show after you start the O2 flowing. Probably the most important, it can give you a head start on shock prevention. I am not a certified EMT anymore, but I found some O2 cylinders and requlator at a garage sale one day (some oldster had passed on), and try to keep it stashed away in a corner of our vehicle...