I would say that the introduction to service manuals are a preliminary service manual for technicians. Some of the best descriptions of operation occur there.
Unfortunately to get good coverage one must have all the manuals of a model up to the year in question and sometimes others. This is because the system is explained well the year it comes out but probably never again. Thus CIS is best discussed in the 1976 manual. The 1992 model years 140 manual is probably the best stuff around on 140 short of WIS. but if one had a 1998, one would want various years to get whichever year the engine system for a 98 came out. The 1996 had a whole book on the 722.6 auto trans.
Bosch Master, ASE Master, L1
33 years MB technician