There hasn't been any proper (publicized) code-diving on the matter as far as I can tell. From a subjective point of view based on experience alone, the int gain from individual scrolls typically depends on the character's aptitude for int gains in general and the value of the given scroll or book. Getting a stat point from reading a SoW or SoCM is much more likely compared to SoDM or even SoEA/W. There may be some sort of diminishing return on stat gain from reading in general as your int goes up, but don't take my word on that.
I may of course be completely wrong and everything readable/usable has a distinct value for stat gains in general.
There was a book of the pantheon or something akin those lines in one of the mods that gave a large boost to both wisdom and int, anyone recall how that boost worked?