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From Harry Potter and the Goblet of Fire:
Harry didn't say anything. Other people might not understand why Uncle Vernon was making a fuss about too many stamps, but Harry had lived with the Dursleys too long not to know how touchy they were about anything even slightly out of the ordinary.
It's the same as 'to not know', or 'to be ignorant about'.
Anyone living with the Dursleys for a certain period of time will know how touchy they are (about anything slightly out of the ordinary). Harry had lived with them for at least that amount of time, so too long not to know about it.
Constructions involving too can be complemented with a non-finite clause, and the verb in the clause can be negated:
The box was too high to reach.
That riddle is too hard to guess.
The ice was too thin to walk on.
The butter was too hard to spread.
He had lived in the town too long not to know that the Mayor's speech would drag on for half an hour.
The child was too old not to know better.
