I've got a complaint about the topic Common Spells.
It is too RNG dependant for being a mandatory research.
You've got to fight and kill (not capture) an enemy mage or priest. Ironically, this makes them worth more dead than alive once you have exhausted their mandatory living topics, but on it's own, it wouldn't be a problem.
You have to hope they drop a Magic Book (14% from Priest, 12% from Mage, let's say 13% average).
You have to hope that the Magic Book doesn't turn out to be a Rune Book (79%, 10.27% cumulative)
You have to hope that whatever book it gave you will give you common Spells. (starts at 4/17=23.53%, 2.42% cumulative, limits to 4/4, 10.27% cumulative).
So you will have to probably go through at least 20-ish dead magi/priests to get it. Which doesn't sound that bad, but you also need them alive for plenty of topics, and they are somewhat rare.