I haven't read the book yet and the main reason is because of the clips of seen of his documentary, he sounded like a random shooter or street kid the cartel members hire by the thousands. Another cannon fodder the cartels employ by the thousands to be their front men but got lucky enough to tell his tale. I'm not interested in the cannon fodder or the dirty work itself but the core group behind that employ these street gangs to either sell drugs or manage their drug houses, act as "whistle blowers" or do street surveillance/intelligence or "halcones" as they call themselves, or sometimes as "sicarios" or hitmen. The reason why the "sicarios" are not as trained as say a couple of years back is because the traffickers themselves are not the shooters anymore. At the start of the Felipe Calderon administration many traffickers were being killed in confrontations with the authorities. So they started to evolve into more sophisticated groups. So they started to hire the local street gangs to be their front men or cannon fodder for them which "El Hummer" was the pioneer of it and all cartels followed suit. Not all traffickers took up a more of administration role but Los Zetas being the most noticeable one, they still kept street presence more so than other cartels. The "Fight for Monterrey" article from Insight Crime anaylsis gives a good insight on the Los Zetas organization and the fight between the Gulf Cartel for that metropolitan area. It basically states that there are 300 "Zetas" in that metropolitan area, and that in each neighborhood there are about 8-10 "Zetillas" or per "Zeta". They make estimations of much or what rackets they are in and how much they get out of those rackets. Many of the dead between those groups were the the local street gangs either cartel hired.

Edit:Here is the link if you're interested http://www.insightcrime.org/zetas-monterrey/how-the-zetas-took-monterrey I recoomend to start with part 1 and go from there.