If I either:
A) Link a new Mystcraft linking book in the nether and leave
or B) Link a new Mystcraft linking book in the overworld, go into the Nether, and leave the Nether again
the game crashes.
I'm not entirely sure what causes this.

EDIT: This appears to happen for other reasons when I spend too long in the Nether and then go back, in the modpack I'm using, so it may be some other mod. Ignore this until I figure out what mod is causing it after all.