This find comes a day late, but I think people will enjoy it none-the-less.
I was searching Google Books today and came across this gem: Kauai: 100 Years in Postcards, By Stormy Cazad, published in 2011. It is part of the Postcard History Series of Acadia Publishing.
The book is full of postcards going back to the early part of the 1900s. It has many views of agriculture and historic buildings, as well as postcards showing people.
Though the book is not available in ebook format, it can be viewed in its entirety on Google Books. Check it out if this island interests you.