Which is correct...
He sought help in a number of punblications that the English had/have written on Ancient Greece.
Albertino is correct for casual/spoken English. This is the way most speakers of English would actually say this sentence.
However, the 'correct' by the book answer is:
He sought help in a number of publications that the English had written on Ancient Greece.
The writing on Ancient Greece happened first, so we have to use past perfect tense (had written).
I just wrote up an article on past perfect tense that has some more examples and rules.