Well, I guess I'm about to maybe hijack this thread...
Service manuals: With my old rigs, a '72 Blazer that I kept for over 30 years, a couple of late 70's VW Rabbits, even a Peugeot 504D Estate (wagon), and my several "vintage" Japanese motorcycles, Chilton's

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or Clymer's

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manuals gave me all I needed to know. (And still do, for the bikes.)
Beginning with our 1987 Plymouth Colt Vista AWD, then with its replacement, a 1993 Plymouth Voyager AWD, then with that replacement, wife's current 2004 Toyota Matrix AWD, followed by my 2005 Jetta TDI Wagon, I ordered the factory service manuals with each & every one.
In every case, they paid for themselves very quickly, even as they became more and more expensive -- the latest ones going for well over $150
each. Still, they paid for themselves, like when I was told "you need a new transmission-- $2000" on the Colt Vista (a broken wire to the vacuum-operated 5th/reverse plunger), to the "you gotta have a major wiring issue, all switches check out OK, another $500 for diagnosis" on my Jetta (a flaky clutch switch, $18)....
But then it got weird: in addition to the service manuals for the Jetta, I needed to purchase proprietary software to work with the Jetta (VAG-COM), ANOTHER $280... but yes, that paid for itself, too.
Now we have, purchased new in 2015, a Roadtrek SS Agile RV, based on the Mercedes Sprinter short/tall chassis, with their OM642 V-6 3.0 diesel. OK.... it seems that, other than a generic code-reader, I'd need the (again, proprietary) M-B software package for complete/thorough/accurate diagnosis of the EXTENSIVE computer-controlled... well,
everything!! Cost is around $20K.

Forget that!
So that's the way it going, apparently... the days of the DIY/backyard/shade-tree mechanic are rapidly coming to an end, methinks. (Remember that scene in the original Star Wars movie where Luke is working on a wicked complex droid like it's no big deal?) That Mercedes is so %$% complicated... I'm not sure if I even WANT to work on the thing, for fear of screwing up some small bit that will end up costing thousands later (yes, it happens, apparently fairly often)... So it looks like I'm stuck with going to a dealer or to an independent --who has the tools and the knowledge-- and are, unfortunately, fairly hard to find....
So, the "new" service manuals, at least for some vehicles, are these forums, where folks can share their experiences and solutions. (I visit the Sprinter forum on a daily basis.)
But, for things that --so far-- aren't totally controlled by a computer and a bunch of sensors.... a good factory service manual is a very sound investment.
Alvin Toffler strikes again....