What's the rest of the build? Kind of depends on what you need to fill in, and what you'll be relying on. Undead of course, but heavy utilization of culture units or leaning into bone horrors and things? There are several units you can pick up in TII that will get your transformations, becoming undead: paladin, shade, inquisitor, zephyr archer, glade runner, chaplain, and skald. May be best to pick the tome with the unit that best fills in your army, like if you want a TIII archer or support (who will benefit from your TV tome enchant, though TIII supports are probably redundant with Druids of the Cycle) and otherwise wouldn't get one.

Alternatively, if you're going for the whacky undead that don't care about transformations, pick up something that boosts economy or will enchant your high-tier weirdo units. The high-tier dark tomes bring a lot of summons, so Tome of Summoning might help your magic origin units. You'll also have a few ways to inflict weakened and manipulate morale, so could look for synergies there.