We've spent months focused primarily on the generals, neglecting pretty much everything else we wanted to do as the invasions had a limited time frame. All in all, we have about 500 books specifically from the generals (mage and healer), using stealth mages and EVs to enable specifically targeting these NPCs without succumbing to the rest of the accompanying spawn. Mostly I played without the wife along, and it would take up to 30 minutes to kill one, not including the time required to separate spawn and get in position not to die. With both of us present it took about 15 minutes to drop one. Some of the spawn sets had multiple mages or healers, many of the spawn sets had none.
The SDI 30s really aren't that rare, I don't have them separated, but theres probably 50 or so of them - about 10% seems right. About 80% of our books are from mages, we prioritized them over all others. On the mage books, the ones with DCI, we have only 5 with both DCI 10 and SDI 30, I'm sure of this. It works out to about a 1% chance for a max "mage book," what I don't know is if that's normal or just prolonged ill fate and bad luck. Max healer books, of which we also have exactly 5, works out to about 5%. We probably have 20 or so DCI 10 books not at max SDI, so the likelihood of a max DCI book seems about 5%. If you're out and about shopping for these, maybe the "math" can help you determine if one is fairly priced, so I'm passing along the numbers for what they're worth!