Ah, thank you for the replies. I also just found a "High Tier Blank Witchcraft Book", which also doesn't combine with the Bone Cage scroll. In fact, I don't even get a 'Combine With' option for that one, I'm guessing since I don't have someone who started as a witch (though I do have someone who has both Scoundrel and Necromancy - it seems odd that I don't get the 'Combine' option, but maybe a certain level of each is needed for the 'High Level' books).
Either way though, Bone Cage is a regular old Necromancy spell - I'm stumped as to why a blank Necromancy skill book won't work with it, unless it requires a "High Level Necromancy Book", which I haven't found yet.
Last edited by skabbo; 04/10/17 02:29 AM.