"extensive letter" is a correct and usable phrase in written English.
You can use it to refer to a letter that is long or detailed. For example: "I received an extensive letter from my aunt about her trip to Europe."
I did however send them the extensive letter that the poor old bastard they tried to "attach" me to (on every level :) at Hampstead Heath sent me.
I wrote an extensive letter to PBS, explaining why this is ethically wrong and why this is unnecessary and why it caves in to Turkish government denial". Mr. McCarthy, however, said PBS was right to provide a forum for opposing viewpoints.
Through an extensive letter writing campaign, Booth was able to bring the total number of Cups at Columbia to 12. Of the remaining 3 cups: one could not be found but the silver engraving was secured (Cup of 1864); one was willed from father to son and was on loan once at the University (Cup of 1868); and only one was completely lost and unaccounted for (Cup of 1871).
Reinhardt, also an economist who earned his health care bona fides long ago, responded to the critics with an extensive letter to the editor.
When we first learnt she'd won the competition and we saw her in clinic we wanted to ensure that it was safe to treat her, that there were no welfare for the child issues, so we contacted her GP who sent us an extensive letter outlining her history, contacted the other medical clinicians involved in her care to find out their feeling on her pregnancy risk.
Virginia, with editorial oversight from her father, wrote an extensive letter to her cousin in Illinois about "our trubels getting to Callifornia".
In addition to the telephone appeal, an extensive letter-writing campaign is being instituted, so voters can express their support for McCarthy to Democratic county chairmen & to Convention delegates.
In an extensive letter-writing campaign, his fellow inmates had described "The Gov" to the judge as "humble" and a "good man" who is knowledgeable and a great teacher.
Polly also wrote extensive letters regarding her own travels between 1953 and 1962.
There are also extensive letters written by Polly Root Collier and Henry Stanton Monroe, Lucy Monroe Calhoun's niece and nephew, both of whom wrote letters to family members during their stays in China.
Writings include letters by Polly Root Collier and Henry Stanton Monoe, Lucy Monroe Calhoun's niece and nephew, both of whom wrote extensive letters to family members during their stays with Calhoun in China, in 1911-1912 and 1933, respectively.
