Pact of the Tome gives Warlocks access to any 3 cantrips. The feature doesn't specify whether the cantrips use the default stat (my guess), or whether they use Cha (to make the feature more useful/avoid MAD).
Contrast the omission with Magic Initiate, which explicitly calls out the stats, according to the class. So, which is it?
Actually, I think I found the answer by re-reading the spells (say, Shillelagh):
The wood of a club or quarterstaff you are holding is imbued with nature's power. For the duration, you can use your spellcasting ability instead of Strength for the attack and damage rolls.
Notice it doesn't say Wisdom (as in previous editions, where spells referenced the specific stat). Since the Warlocks' spellcasting ability is Charisma based (and the spell is gained via a Warlock feature), that's what the spell uses.
Thanks to an answer below, here's an unofficial ruling.
This is clarified in the PHB Errata (v1.1):
Pact of the Tome (p. 108). The cantrips are considered warlock spells for you, and they needn’t be from the same spell list.
By adding from PHB page 107 that:
Charisma is your spellcasting ability for your warlock spells
we can conclude that these spells also use Charisma as the spellcasting ability.
There is no default ability for most spells.
Each class sets it's own spell casting ability. Thus for a Warlock, the proper ability is Charisma.
If you multiclass and gain spells from both classes, you use the spell casting ability from the class from which you gained the spell.
The spell casting abilities for each class are listed in the description for the class, in the Spell Casting ability section. Note that in chapter 10 it regularly refers to your "spell casting ability" and the following text is repeated throughout the character caster descriptions:
Use your [ability] whenever a spell refers to your spellcasting ability. In addition you use your [ability] modifier when setting the saving throw DC for a [class] spell you cast and when making an attack roll with one. (extracted from Paladin, on page 85 of the PHB).
You should use the Warlock's CHA for any cantrips gained via Pact of the Tome, according to this tweet from Jeremy Crawford:
"The intent of Pact of the Tome is that the chosen cantrips become warlock spells with Cha. as the spellcasting ability."
