A quick breakdown on what does what—
AMOUNT is the intensity of the effect being applied. In this case, it's best to start with this set to 500%, not because you'll EVER use it there, but because it'll help you judge what's happening better.
RADIUS is the width of the sharpening effect. If this is set too high, your fine-line information will magically become thick-line information, and the same for your dense areas of hatching. If this is set too low, the effect will be minimal, and might only contribute noise to your image. If you have a very good scan and it's been only minimally (up to 250 percent) upscaled, then the Radius should probably be set somewhere near 1 pixel. Let's start it there for this application, and work it up if we need to.
THRESHOLD is how deep the effect goes. How contrast-y does an area have to be for contrast to be applied? If you have the Threshold set too low, then Unsharp Mask will pick holes in your black areas and make noise like unerased pencil lines or smudges or dirt or... whatever... more prominent. If you have the Threshold set too high, then you'll only be affecting the areas of your image that are already the most contrast-y, defeating the purpose of the sharpening in the first place.
Okay, so I've set my Amount to maximum (500%) and my Radius looks good at 1.1 pixels and Threshold at 20 or so. I can see my details firming up, my fine hatching opening up, and scooting across and checking the rest of the image, I can see my finest lines firming up as well. Once I have the other settings, I back my Amount down to a more reasonable (and less crispy-looking) 200 percent or so, and hit Okay.
Now, I'm going to take a look at different parts of my image and see what I've done, by turning on my Threshold Adjustment layer again and clicking the Sharpened layer on and off, so we can directly compare the sharpened and unsharpened image. I see that lots more of my detail has been retained, both fine line and dense hatching, including some unintentional "details" that I'll have to clean up later (like the razor blade cuts where Gerhard used and exacto knife to trim the mechanical tone!) But it's looking pretty good overall.
But we can improve it just a bit more.
Now that we've done our first round of sharpening, I'm going to knock out the remainder of our paper color by making another Levels adjustment. Hit Ctrl-Alt-L to bring up the Levels command.
In my case, I'm going to leave my Black Point and Mids Point/Gamma Control exactly where they are and only move my White point just a bit to the left, knocking out a bit more of the "color." This will enable us to retain detail within/underneath the tone as well, which is after all just a bit softer and filled-in because of the softening presented by the carrier film of the tone. (if your tone is yellowed and aged, then this is even more critical). Just a little bit will do.
After this, depending on the softness of your scan, you might require one more blast of sharpening, possibly with a different radius than the first. As always, compare your results as you go, and play a little conservatively with this, keeping in mind we're making a script to be appropriate to every page scanned from original art!
In my case, the page looks pretty good here, so I'm going to stop my script and call it good, knowing that I might choose to sharpen a bit more on some areas of some pages in my cleanup stage.
So, how did we do?