The books have their own ideas about the meaning of the word 'unique'.

I agree for all those cases where it's possible to obtain something from multiple sources and where the object in question is something that is kept and never destroyed. So for all the cases you mentioned it's fine. I like to think of those things as attributes your character has, or knowledge your character has.

For books however, they are physical, so if you can get the same book from two places, then two copies of the book exist in game world - therefore you should be able to have two copies of that book on the table.

It seems extra strange that it's possible to have two books for the same language, while you cannot have two copies of most of the other books (since only language books, upgrade books (for passion and reason), and the book that signifies the store is empty of good stuff are non-unique).

I suppose multiple copies of language books exist as a failsafe mechanism so that player has access to them in any case? However, to be honest, to lose all access to languages you have to first auction all the books from the bookshop, then ignore books at auction house, and then murder your patrons with lantern-winter spell, which doesn't sound like something that can be done accidentally (and, in fact, al-Adim is immortal so you will never lose access to Aramaic). Also, I suppose, language books, unlike occult ones, are printed freely and thus exist in many copies as opposed to very limited amount.
Making them unique, however, makes no sense. Even if you have spare copies of said books, you can auction them, so they are at least worth money.

Exactly - so it's annoying that you can auction off spare language books, but you cannot do this with the "unique" books - at least not if you get one at the same time as already owning one. It's currently possible to auction spare "unique" books only if you get one while there isn't currently a copy of it on the table (ie you've either already researched another copy of it, or it's currently being researched or auctioned) - which makes no sense.

So either the "unique" books should actually be "unique" - that is, a single copy only available in a single playthrough - or they should simply lose the "unique" attribute and behave like the other non-unique books.

Well, the last update has made language books and Weary Detective non-unique at least.

I'm not sure if this is an old bug or a new one but - when you use a summoned creature to summon its copy, like with Maid, and that summoned creature rebels and then you release it, the first one's timer is reset. Which, in turn, means that even if you don't have the second copy, you don't need it that much anymore.

Maybe that means it is the first one that rebels and you keep the new one.

Doesn't sound like a good explanation considering they generally are absolutely obedient once summoned.

programming-wise, I mean.

Didn't even know raised corpses (as opposed to other summons like Maid in Mirror) could rebel in the first place - maybe they're not supposed to and that's why this happens?

Speaking of raising corpses, is it just me or is it much easier to summon the proper summons (Hint, Percussigant, Maid, Prophet) as opposed to raising a corpse? So far I've only bothered to raise a corpse once, and I only figured this out after I'd already been summoning the aforementioned things comfortably. That and those things don't require a corpse, which is more time-consuming (and risky) to acquire than the stuff needed for the other summons.

From their stats, it appears raising corpses was originally designed to be done earlier in-game than the other summons.

I honestly use corpse raising solely as means of disposing of a corpse (though you can dispose of it easily by using it in cultist promotion as well).
Even if it was easier to perform them than regular summons (like, if getting a corpse wasn't itself unreliable), what could you possibly do with a 4 moth 4 winter minion? You'd need like 3 of those for expedition and sending them on a mission is worse than sending a hireling.
And that's all not to mention their timer being even shorter than that of summons.
Corpses aren't even cool flavor-wise compared to something like Hint or Maid. Those are amazing.

OK so I guess most other people also raise them less than using other summons.

Agree that the stats are crap - but it's not absolutely necessary to have huge stats for the expeditions, it just makes success more likely (and take less time/money). They can be supplemented with other corpses or even minions easily enough.

I've succeeded at a number of the easier expedition challenges with the required stat at only 4 or 5 plenty of times - if they were significantly easier to raise than the other (better) summons, then they could be useful in the early game when you don't have many cult members or cannot raise the good summons yet. As it stands though, it's generally easier and faster to find and hire a hireling or two instead (despite the randomness of getting hirelings, they're still usually faster to get than a raised corpse).

Alternatively, if all summons required a corpse, that could balance things (IMO if they do this then most/all rites should get an extra "space" for a corpse or similar item, otherwise summoning will be too annoying).

Sure, you don't have to have 10 for expedition but when it's so easy to get (hint + 2 edge/lantern, maid without any help at all. or just 2 5-stat cultists) you don't really have a reason to waste time and resourses with sub-10 expedition party. And in early game when you don't have those summons, expeditions are ill-advised as you don't usually have good ways to deal with hunters either.
As for using corpses for all summons... it doesn't sound that good. It doesn't fit the flavour at all, and would make the process of summoning a lot more annoying. Plus, it kind of ruins the formula of rite being defined by the sum of aspects (that makes an exception for corpses now anyway, which is not good in my opinion).
But what can (along with making the rite more easily accessible) redeem the crappy-statted minions would be, in my opinion, varying difficulty on challenges. Say, expeditions in the city would require less than 10 in according stat for guaranteed success, or lower-level evidence would be easier to destroy, or hunters, depending on their physical power, would require different amount of edge to defeat.

That... is actually a good idea. Though if it were significantly easier to raise them, then one could just raise a bunch before starting an expedition, and just send in 2 straight away, then another one if needed.

TBH I'd probably also find it too annoying if other summons required bodies.

What I wouldn't mind as well is the ability to do "cult business" with raised bodies, so we could use them to capture random people, burgle, or build things (for some reason I just started thinking of 1400: The Guild, where as the Necromancer profession people would lease funeral plots from you, then when they died you could raise them as zombies who would build things for you. The zombies also sold life insurance...).