No more links, but I do have some thoughts. If this is supposed to be a sidekick party support type character, she'll work fine. But if you want her to operate on an equal footing with the party, or serve as an adversary later on, you're going to want to use a stronger class. Mystic Theurge is a pretty weak class, and True necromancer is a weaker version of it - trading even more caster levels away for the ability to imitate spells that a single classed character could have already cast several levels ago. On top of that, most of her PrC abilities are going to be pretty useless if she isn't actually making or using undead.
Again, if this is purely an npc support character, that's fine, since you don't want them overshadowing the heroes anyway.
Another problem you might run into if she's trying to hid what she is, though: None of those classes have disguise or bluff on their class skills list, iirc, and even if they did her cha is probably going to be pretty low to accomodate a higher int and wis, so she might not be that good at hiding it. If she worships Nerull she can have Trickery as her other Domain, which could help this a little.
Yet another potential problem is that Mystic Theurge type characters can be a lot of hassle and book keeping, even though they're pretty weak compared to single classed casters. You've got a lot of spell slots to keep track of day to day, and a lot of different spells that could go into them. She'll be a lot of work.
If, after all of that, you want to stick with with a True Necromancer, be sure to pick up disguise self (either from the wizard half or the trickery domain), and focus on buff spells to augment the party.
If, on the other hand, you want to consider some other options, here are some rivals for the True Necromancer:
Mystic Theurge (DMG): Shares many of the TN's problems, but not quite as weak, and simpler.
Dread Necromancer (Heroes of Horror): My personal favorite base class. Warmage-style spontaneous casting of his highly focused, highly fluffy spell list. Disguise and Bluff as class skills for a Cha based caster. Damage Reduction and self healing (if undead or having the Tomb Tainted Soul feat from Libris Mortis), mean the party don't have to worry about her or spend resources on her. Can have an imp or Quasit familiar, either of which is capable of taking on an animal form to avoid suspician. Does a reasonable impersonation of a touch spell leaning sorcerer gish, keeping the party and others guessing. At mid to high levels, the character's undead animating ability positively explodes, allowing the character to quickly make a conversion to serious villainous threat if desired. Simple (for a caster), fluffy, subtle, and effective, while being less powerful and less paperwork then traditional casters.
Archivist (Heroes of Horror): a fluffy class that combines dark knowledge with wizard-like casting of the cleric spell list. Can also learn non-cleric list divine spells if he finds scrolls of them to add to his spell list, allowing you to give him fluffy sorcerer or wizard spells via divine scrolls of domain spells. When you consider that, he basically becomes something like a mystic theurge, but all on one caster progression. No rebuking, but proper 'prayer books' go a long way towards conveying that necromancer feel.
Ultimate Magus (Complete Mage): If what you want is a duel caster class that actually works, this is the closest you'll get. Three levels of wizard, one of sorcerer (or beguiler, or dread necromancer) with the feat Practiced Spellcaster (Complete Arcane) for the spontaneous end nets you a dual progression character that loses very little caster progression from the wizard side. Plus you get some fun and pretty effective metamagic tricks to make combining two different progressions meaningful.
Plain old Wizard, Cleric, or Sorcerer. Proper feat, spell, or domain selections can give each of these classes a necromantic feel without bothering to dip into PrCs or non-core classes. And, to be honest, single classed wizards, clerics, and sorcerers with proper spell selections are also just inherently stronger then Mystic Theurges, True Necromancers, or Dread Necromancers, due to having a wider range of higher level spells.
There are other good options for necromantic characters, including Pale Master and Master of Shrouds (Libris Mortis), Necrocarnate (Magic of Incarnum), Bone Knight (Eberron: Five Nations), and more. 3.x had dozens of options for necromantically inclined characters. But most of those, like True Necromancer frankly, seem better suited to an overt necromancer, rather than one trying to hide what they are from friends and strangers.
