Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

The helping verb had
Consistency of verb tense
Revising at the word level

The helping verb had is used when you are writing in the past tense and you wish to indicate something that happened even earlier. For instance, these are correct examples:

(a) Samantha started breakfast at seven o’clock this morning; by that time, she had read the morning paper and washed two loads of clothes.

(b) Frank had visited France three times in the 1970s and 1980s before he made his first trip with Catherine last year.