Lyndon Cook's has a provocative new book out on the Seventy during the Joseph Smith period. It's called "A Tentative Inquiry into the Office of Seventy, 1835-1845." It's a great read for history and doctrinal buffs.
For the first time in Mormon scholarship, Cook satisfactorily elucidates the early notion of "high priesthood" — what it was and what it was not; who had it and who did not, and what its relationship was to priesthood office and to the order of Melchizedek itself?
For not unlike their ministry at Kirtland and Nauvoo, the high priests in Utah and the west were the "standing ministers" of the church (i.e., the bishoprics, stake presidencies, and high council). Apparently, no amount of sermonizing could change that or position the seventy over the high priests in the minds of the faithful. Cook says that in spite of the official rhetoric or the revelations, the seventy have been the step children of Mormon priesthood almost from the beginning.
Available at http://www.watsonandwatsonbooks.com/product.i?sku=0001