[satiric]

I bought a Fluke 8840A bench multimeter recently. Its manual has comprehensive electrical schematics of the internals! Not only that, a significant portion of the manual is taken up by explaining exactly how each portion of the circuit works. Mine didn't come with the AC option, but if I wanted to I could build one without having to reverse engineer the schematic. As someone who didn't grow up in that era (was born in the late nineties) that's pretty wild to me.

[silisili]

In that domain, also, the Swanson speed square. Comes with a 'blue book' full of all kinds of tips and tricks and measurements.