The Black tome is straightly harmful. I once found it in a power surge somewhere in ToTHK and took it to Dwarftown. It costed me ~3 corruptions (yes, I got the last one in Dwarftown itself). Next time I will live it alone.
Well, consider the following situation.
You are a grey elf monk and a teleportation trap sends you to a portion of the dungeon that is closed off by a huge river. Even when you manage to cross the river back, you will have to fight off a bunch of out-of-depth monsters that you can't handle yet.
The only spell you learned so far was Strength of Atlas. You didn't learn to swim yet and you do not have any items with you except for the cursed SoCR from the HMV, 10000 potions of plain water, 500 cooked lizards and 500 ratling pamphlets.
The floor you end up on is littered with countless logs because.. err.. because it is a special new dungeon. It also has a coaligned altar.
Now, what is the one artifact that will save you? You guessed it: the black tome.
You steal a scroll labelled HITME. The orc hits you.
Well, you can create blank scrolls with water + ratling pamphlets. With black tome + water you can create potions of raw chaos, and with potions of raw chaos and logs you can safely create random tools. Some of these tools will be writing sets (provided the danger level is high enough). Monks can write SoCR without mana penalty, and with so many writing sets and blank scrolls (all blessed) they can write lots of SoCR.
Which, in turn, allows you to read the black tome for random spells. Eventually, you should learn Frost Bolt and enough offensive spells to deal with any monsters on the level.
You steal a scroll labelled HITME. The orc hits you.
Black tome found early yields a pretty decent price when sold.
It's useful when you need to get multiple corruptions on D:50 to enter chaos plane.
In case you can't use moon sickle for whatever reason, black tome works fine as PoRC generator.
I frequently play around with chaos alchemy and on some few, rare occasions I have somehow missed moon sickle on my route through CoC and instead found the black tome at some point.
All this though doesn't change the fact that everything about this artifact can be replicated with more mundane items, most of which are guaranteed.
It's still the worst artifact out there.
"Hell is empty and all the devils are here."
From my experience stone giant corpses act as they used in 1.1.1 and not like other giant corpses now [I've seen increase from 28 to 29, and increase rate is as expected].
One would need food preservation/weapon of hunting, but potentially hammerhead can allow to pot some work on St for real.
Hammerhead
It's not really clear whether the question is to rate the artifacts relative to eachother (given the fact that you find an artifact, how would you rate x?) or rate them 'in general'. In the latter case I'd rate almost all of them 'good' or above but judging to the post above it's the former way of rating that's intended. Maybe state it more clearly in the question form?
Nice to see the overview on the spreadsheet. Artifacts that can be situationally useful are spread way more over all categories (Silver Key, Black Tome, Shezestriakis, Celestrix, Ring of Immunity, Staves) than artifacts that are always useful (missiles and missile weaps, Robes of Resistance/Nature's Companion, Bracers of War etc.). Justifier is spread all over the place since it depends completely on class
I think Grod and Kinslayer are pretty exceptional and undervalued! I also rated the Silver Key as exceptional, simply cuz I never had it and I can imagine its useful, especially early game. It's in some way unique among the artifacts as it improves no stats or fighting skills at all nor does it offer any protection yet it can still help you alot if you remember to use it.