I do think that there should be more requirements for learning a secret. Right now, if a necromancy book shows up in your fort, the knowledge in it spreads like a virus.
After an otherwise stable and uneventful fort became unplayable after half the population learned how to throw fireballs and the other half learned how to raise the dead, I have to concur. Learning how to make dead people get back up, shoot fire, or whatever else you've had the hubris to mod in has always been as easy as opening a book. Now that those books are easy to come by and usable by any schmuck what comes across them, I find myself flooded in more necromancers, wizards and more than I feel I ever should have had to deal with. Right now I feel like my choices are to get rid of any interaction books that pop up or let it happen, without any kind of middle ground. Something's got to be done.
I think that whatever else goes into fixing this, learning an interaction from a book has to be an ultimately voluntary action, as in you shouldn't learn the secret the moment you open the book. In adventure mode, you pick up a book, you read it, and you're told something like "This book contains the secrets of life and death. Keep reading? Y/N". In fort mode, a dwarf picks up a book, they read it, and they only decide to learn the secret if they have the correct goal, they don't keep going and learn necromancy unless they're after immortality.
The adventure mode prompt keeps you from gaining a power you may not have wanted just because you didn't know about the book beforehand, and the fort mode goal check keeps your dwarves from becoming necromancers unless they were going to try for that anyway. Learning the secret is still possible in both situations, but it's ultimately up to the individual if they go all the way.
I'm quite curious about how divination would work in DF, thought it's sort of in a "I have a theory, and I really want to see how right or wrong it is" sort of way. Since so much of the game is randomly generated, I figure that divination would merely be randomly generating those events ahead of time and telling you of them in advance.
Predictions for volcanic eruptions and earthquakes would probably be easy to implement. Major seismic events are set in motion long before they actually occur, predicting them would be as easy as giving the soothsayer a sneak peak at a countdown for an event that was already going to happen anyway. The way I see it, they would essentially be fixed points determined in the early stages of world gen at their simplest, but Toady is probably going to want to go with a more convincing model for geological activity when the time comes.