Spells in lair reward books?

[Seravy]

How are they selected?

I found 2 chaos books (one at a time), and Disintegrate which is very rare and should require 3 books is available for research. (I had no chaos books at the start of the game)
I didn't swap that spell's position or rarity, so I wonder, is this a bug or an intended feature? Do found books give spells of random rarity, regardless of the number of books? I thought they follow the same pattern as books at the start of the game but maybe not? I also found Chaos Mastery, Sorcery Mastery and one additional Sorcery book but those are most likely not related.

[Tiltowait]

It's not a bug, researching a spell and receiving a reward from a lair are different.

The reward a lair gives is dependent on the strength of the monsters inside, as expressed by the number of mana required to conjure them. See the Official Strategy Guide for details.

[Anthony]

That doesn't seem to be the issue -- when you get a new spellbook, it adds spells to your researchable list, and apparently it added disintegrate, which doesn't make sense.

[Tiltowait]

Oh, that wasn't really clear, "found 2 chaos books and disintegrate" sounds like the spell was his reward from the lair in addition to getting the 2 spellbooks. I don't know if it's documented that getting new books uses the same function to add new spells to your research list, or is it?

[Seravy]

Oh, that wasn't really clear, "found 2 chaos books and disintegrate" sounds like the spell was his reward from the lair in addition to getting the 2 spellbooks. I don't know if it's documented that getting new books uses the same function to add new spells to your research list, or is it?

Seems like I could have worded that better. Yeah the problem is I was able to research Disintegrate with 2 Chaos books only.

Edit : found 2 nature books in a node and this time I immediately went to see what spells the books unlocked. (had 0 Nature books before node)
Received 5 commons, 2 uncommons, 1 rare and 1 very rare for a total of 9 spells from the 2 books. I'll try to find the code for receiving books to figure out how it works.
hmm...not there yet but I found that if you get a failed special, and the lair already contained 3 items, you get 1000 mana instead.
Also, that there was a 3rd tier of Specials, but is now disabled, 1000 gives you specialx1, 2000 and 3000 both specialx2 instead of 3000 being specialx3.

Found it!
The function works like the following :
If it's the first book of that color : 3 Commons, 1 UC added to research.
If second book : 2 Commons, 1 UC, 1 R, 1 VR
3rd-6th books are 1 of each rarity.
7th book is 1 C, 2 UC, 1 R, 1 VR
8th is 2 UC, 1R, 1 VR
9th and 10th is 2 R, 1 VR

In other words, book number 2 can and will contain exactly one very rare.