I'm sure many mechanics have good knowledge of bolt arrangement, but I also bet their knowledge is based on experience where they've seen several methods; some resulting in being a weak point where they know to use a stronger bolt.
Mechanical engineers know some good in depth knowledge of materials.
My point is that it depends on the level you want to learn. If you want to know names of bolts/washers/nuts, a mechanic is great. If you want to learn weak points on various cars, which bolts need improving because of these weak points, tools to remove/install parts, and methods to remove/install parts, a mechanic is no doubt your best option. If you want to spend a few weekends learning materials, tension, torque, etc... then reading is your best option.
As I mentioned before, if I remove a bolt on a car or power equipment, I check the bolt/nut for age followed by its application. A bolt the diameter of my finger is most likely not worth replacing unless it's on suspension and worn down due to rust. Anyway, my point is, if you're working on a car that's a year or two old, then you'd need to be a mechanical engineer to know whether you should change bolts because they may break in time. If you do that, you'll spend quite a bit of time analyzing all the forces, materials, temperature variations, etc...
If you work on a car that's ten-years-old and you don't see any bolts that are broken or worn out, chances are they will last because they lasted the test of time. If you're working on a twenty-five-year-old car or longer, it's either a piece of junk and you'll be replacing only the necessary bolts, or you're restoring it in which case you'll want to replace bolts.
My approach is take the bolt/nut in question and measure it. Get the basics, is it a hex bolt, how long is it, diameter, and thread (TPI if it's SAE or pitch if it's metric), go on McMaster Carr, and start with basic filters. i.e. hex, 1" long, 3/8" diameter, 24TPI (assuming 3/8" bolt comes in 24TPI).
Now, McMaster will show you the remaining options for a bolt with those specs. Do they have options for stainless steel, different grades, etc...?
You have the option for regular or stainless steel, research the differences like tensile strength and whether it's corrosion resistant.
As for torquing, every bolt/nut has a torque spec, but it's rare I torque any bolt with a torque wrench. In fact, I've replaced intake manifold gaskets on two cars, same engine type, and the torque spec in Haynes caused the plastic plenum (?) to crack on one bolt hole. A brand new out-of-the-box plenum and it began cracking. The second car I replaced the intake manifold gasket on I began tightening the bolts in the pattern outlined in Haynes with a regular socket wrench, I went around and around until I felt it was snug enough to begin using the torque wrench. To my surprise the torque wrench clicked on every bolt telling me I either went a bit too tight on all of them, or I got them just right just by feel from years of practice. Either way, I did that job about ten-years-ago and the engine is still running without a leak.
Point of my story is: you can read all the books you want, talk to anyone you want, but it also takes feel. When you're tightening a bolt and it begins grabbing or suddenly gets easier to tighten, all the education in the world hasn't prepared you to know that you may be cross threading the bolt (if it got harder) or the bolt is ready to snap or something cracked (because it got easier).
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